subject:"\[Python\-ideas\] Re\: Equality between some of the indexed collections"



On 09.05.20 22:16, Andrew Barnert wrote:

On May 9, 2020, at 02:58, Dominik Vilsmeier  wrote:


Initially I assumed that the reason for this new functionality was
concerned with cases where the types of two objects are not precisely
known and hence instead of converting them to a common type such as
list, a direct elementwise comparison is preferable (that's probably
uncommon though). Instead in the case where two objects are known to
have different types but nevertheless need to be compared
element-by-element, the performance argument makes sense of course.

So as a practical step forward, what about providing a wrapper type
which performs all operations elementwise on the operands. So for example:

 if all(elementwise(chars) == string):
 ...

Here the `elementwise(chars) == string` part returns a generator which
performs the `==` comparison element-by-element.

This doesn't perform any length checks yet, so as a bonus one could add
an `all` property:

 if elementwise(chars).all == string:
 ...

There’s an obvious use for the .all, but do you ever have a use for the 
elementwise itself? When do you need to iterate all the individual comparisons? 
(In numpy, an array of bools has all kinds of uses, starting with indexing or 
selecting with it, but I don’t think any of them are doable here.)

I probably took too much inspiration from Numpy :-) Also I thought it
would nicely fit with the builtin `all` and `any`, but you are right,
there's probably not much use for the elementwise iterator itself. So
one could use `elementwise` as a namespace for `elementwise.all(chars)
== string` and `elementwise.any(chars) == string` which automatically
reduce the elementwise comparisons and the former also performs a length
check prior to that. This would still leave the option of having
`elementwise(x) == y` return an iterator without reducing (if desired).

And obviously this would be a lot simpler if it was just the all object rather 
than the elementwise object—and even a little simpler to use:

 element_compare(chars) == string

(In fact, I think someone submitted effectively that under a different name for 
more-itertools and it was rejected because it seemed really useful but 
more-itertools didn’t seem like the right place for it. I have a similar 
“lexicompare” in my toolbox, but it has extra options that YAGNI. Anyway, even 
if I’m remembering right, you probably don’t need to dig up the more-itertools 
PR because it’s easy enough to redo from scratch.)


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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-09 Thread Andrew Barnert via Python-ideas

On May 9, 2020, at 02:58, Dominik Vilsmeier  wrote:
> 
> 
> Initially I assumed that the reason for this new functionality was
> concerned with cases where the types of two objects are not precisely
> known and hence instead of converting them to a common type such as
> list, a direct elementwise comparison is preferable (that's probably
> uncommon though). Instead in the case where two objects are known to
> have different types but nevertheless need to be compared
> element-by-element, the performance argument makes sense of course.
> 
> So as a practical step forward, what about providing a wrapper type
> which performs all operations elementwise on the operands. So for example:
> 
> if all(elementwise(chars) == string):
> ...
> 
> Here the `elementwise(chars) == string` part returns a generator which
> performs the `==` comparison element-by-element.
> 
> This doesn't perform any length checks yet, so as a bonus one could add
> an `all` property:
> 
> if elementwise(chars).all == string:
> ...

There’s an obvious use for the .all, but do you ever have a use for the 
elementwise itself? When do you need to iterate all the individual comparisons? 
(In numpy, an array of bools has all kinds of uses, starting with indexing or 
selecting with it, but I don’t think any of them are doable here.)

And obviously this would be a lot simpler if it was just the all object rather 
than the elementwise object—and even a little simpler to use:

element_compare(chars) == string

(In fact, I think someone submitted effectively that under a different name for 
more-itertools and it was rejected because it seemed really useful but 
more-itertools didn’t seem like the right place for it. I have a similar 
“lexicompare” in my toolbox, but it has extra options that YAGNI. Anyway, even 
if I’m remembering right, you probably don’t need to dig up the more-itertools 
PR because it’s easy enough to redo from scratch.)

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[Python-ideas] Re: Equality between some of the indexed collections


On 09.05.20 14:16, Dominik Vilsmeier wrote:


On 09.05.20 12:18, Alex Hall wrote:


On Sat, May 9, 2020 at 11:57 AM Dominik Vilsmeier
mailto:dominik.vilsme...@gmx.de>> wrote:

So as a practical step forward, what about providing a wrapper type
which performs all operations elementwise on the operands. So for
example:

 if all(elementwise(chars) == string):
 ...

Here the `elementwise(chars) == string` part returns a generator
which
performs the `==` comparison element-by-element.


Now `==` has returned an object that's always truthy, which is pretty
dangerous.


That can be resolved by returning a custom generator type which
implements `def __bool__(self): raise TypeError('missing r.h.s.
operand')`.


After reading this again, I realized the error message is nonsensical in
this context. It should be rather something like: `TypeError('The truth
value of an elementwise comparison is ambiguous')` (again taking some
inspiration from Numpy).
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[Python-ideas] Re: Equality between some of the indexed collections


On 09.05.20 12:18, Alex Hall wrote:


On Sat, May 9, 2020 at 11:57 AM Dominik Vilsmeier
mailto:dominik.vilsme...@gmx.de>> wrote:

So as a practical step forward, what about providing a wrapper type
which performs all operations elementwise on the operands. So for
example:

 if all(elementwise(chars) == string):
 ...

Here the `elementwise(chars) == string` part returns a generator which
performs the `==` comparison element-by-element.


Now `==` has returned an object that's always truthy, which is pretty
dangerous.


That can be resolved by returning a custom generator type which
implements `def __bool__(self): raise TypeError('missing r.h.s. operand')`.



This doesn't perform any length checks yet, so as a bonus one
could add
an `all` property:

 if elementwise(chars).all == string:
 ...


This is now basically numpy.

```
In[14]: eq = numpy.array([1, 2, 3]) == [1, 2, 4]
In[15]: eq
Out[15]: array([ True,  True, False])
In[16]: eq.all()
Out[16]: False
In[17]: eq.any()
Out[17]: True
In[18]: bool(eq)
Traceback (most recent call last):
...
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
```

I've used number instead of strings because numpy treats strings as
units instead of iterables for this kind of purpose, so you'd have to
do some extra wrapping in lists to explicitly ask for character
comparisons.



Actually I took some inspiration from Numpy but the advantage is of
course not having to install Numpy. The thus provided functionality is
only a very small subset of what Numpy provides.

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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-09 Thread Alex Hall

On Fri, May 8, 2020 at 8:38 PM Steven D'Aprano  wrote:

> On Fri, May 08, 2020 at 07:52:10PM +0200, Alex Hall wrote:
>
> > Would the proposal come with a new magic dunder method which can be
> > overridden, or would it be like `is`?
>
> An excellent question! I don't think there needs to be a dunder.

The problem with this to me (and I think it's part of what David and others
are saying) is that you're proposing additional syntax (for which there's
usually a high bar) for the marginal benefit of improving a very specific
use case.

For comparison, the recent `@` operator is also intended for a very
specific use case (matrix multiplication) but it can at least be reused for
other purposes by overriding its dunder method. On top of that, we can see
very clearly how the arguments in Guido's essay on operators applied to
this case, with clear examples in
https://www.python.org/dev/peps/pep-0465/#why-should-matrix-multiplication-be-infix.
That doesn't apply so well to .EQ. as using `==` twice in a single
expression isn't that common, and any specific flavour like .EQ. is even
less common. `list(a) == list(b)` or `sequence_equal(a, b)` is suboptimal
for visual mental processing, but it's still fine in most cases.

I would be more supportive of some kind of 'roughly equals' proposal (maybe
spelt `~=`) which could be overridden and did sequence equality, case
insensitive string comparison, maybe approximate float comparison, etc. But
even that has marginal benefit and I agree with the objections against it,
particularly having 3 operators with similar equalish meanings.

Perhaps a better alternative would be the ability to temporarily patch `==`
with different meanings. For example, it could be nice to write in a test:

with sequence_equals():
assert f(x, y) == f(y, x) == expected

instead of:

assert list(f(x, y)) == list(f(y, x)) == list(expected)

or similarly with equals_ignoring_order(), equals_ignoring_case(),
equals_ignoring_duplicates(), equals_to_decimal_places(2),
equals_to_significant_figures(3), etc.

This could be especially nice if it replaced implicit uses of `==` deeper
in code. For example, we were recently discussing this function:

```
def zip_equal(*iterables):
sentinel = object()
for combo in zip_longest(*iterables, fillvalue=sentinel):
if sentinel in combo:
raise ValueError('Iterables have different lengths')
yield combo
```

`sentinel in combo` is worrying because it uses `==`. For maximum safety
we'd like to use `is`, but that's more verbose. What if we could write:

```
def zip_equal(*iterables):
sentinel = object()
with is_as_equals():
for combo in zip_longest(*iterables, fillvalue=sentinel):
if sentinel in combo:
raise ValueError('Iterables have different lengths')
yield combo
```

and under the hood when `in` tries to use `==` that gets converted into
`is` to make it safe?

That's probably not the most compelling example, but I'm sure you can
imagine ways in which `==` is used implicitly that could be useful to
override.

I'm not married to this idea, it's mostly just fun brainstorming.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-09 Thread Alex Hall

On Sat, May 9, 2020 at 11:57 AM Dominik Vilsmeier 
wrote:

> So as a practical step forward, what about providing a wrapper type
> which performs all operations elementwise on the operands. So for example:
>
>  if all(elementwise(chars) == string):
>  ...
>
> Here the `elementwise(chars) == string` part returns a generator which
> performs the `==` comparison element-by-element.
>

Now `==` has returned an object that's always truthy, which is pretty
dangerous.

> This doesn't perform any length checks yet, so as a bonus one could add
> an `all` property:
>
>  if elementwise(chars).all == string:
>  ...
>

This is now basically numpy.

```
In[14]: eq = numpy.array([1, 2, 3]) == [1, 2, 4]
In[15]: eq
Out[15]: array([ True,  True, False])
In[16]: eq.all()
Out[16]: False
In[17]: eq.any()
Out[17]: True
In[18]: bool(eq)
Traceback (most recent call last):
...
ValueError: The truth value of an array with more than one element is
ambiguous. Use a.any() or a.all()
```

I've used number instead of strings because numpy treats strings as units
instead of iterables for this kind of purpose, so you'd have to do some
extra wrapping in lists to explicitly ask for character comparisons.
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[Python-ideas] Re: Equality between some of the indexed collections


On 08.05.20 19:01, Steven D'Aprano wrote:


All this proposal adds is *duck-typing* to the comparison, for when
it doesn't matter what the container type is, you care only about the
values in the container. Why be forced to do a possibly expensive (and
maybe very expensive!) manual coercion to a common type just to check
the values for equality element by element, and then throw away the
coerced object?

If you have ever written `a == list(b)` or similar, then You Already
Needed It :-)


Initially I assumed that the reason for this new functionality was
concerned with cases where the types of two objects are not precisely
known and hence instead of converting them to a common type such as
list, a direct elementwise comparison is preferable (that's probably
uncommon though). Instead in the case where two objects are known to
have different types but nevertheless need to be compared
element-by-element, the performance argument makes sense of course.

So as a practical step forward, what about providing a wrapper type
which performs all operations elementwise on the operands. So for example:

    if all(elementwise(chars) == string):
    ...

Here the `elementwise(chars) == string` part returns a generator which
performs the `==` comparison element-by-element.

This doesn't perform any length checks yet, so as a bonus one could add
an `all` property:

    if elementwise(chars).all == string:
    ...

This first checks the lengths of the operands and only then compares for
equality. This wrapper type has the advantage that it can also be used
with any other operator, not just equality.

Here's a rough implementation of such a type:

    import functools
    import itertools
    import operator


    class elementwise:
    def __init__(self, obj, *, zip_func=zip):
    self.lhs = obj
    self.zip_func = zip_func

    def __eq__(self, other): return self.apply_op(other,
op=operator.eq)
    def __lt__(self, other): return self.apply_op(other,
op=operator.lt)
    ...  # define other operators here

    def apply_op(self, other, *, op):
    return self.make_generator(other, op=op)

    def make_generator(self, other, *, op):
    return itertools.starmap(op, self.zip_func(self.lhs, other))

    @property
    def all(self):
    zip_func = functools.partial(itertools.zip_longest,
fillvalue=object())
    return elementwise_all(self.lhs, zip_func=zip_func)


    class elementwise_all(elementwise):
    def apply_op(self, other, *, op):
    try:
    length_check = len(self.lhs) == len(other)
    except TypeError:
    length_check = True
    return length_check and all(self.make_generator(other, op=op))
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-09 Thread Steven D'Aprano

Thanks Andrew for the excellent analysis quoted below. Further 
comments interleaved with yours.


On Fri, May 08, 2020 at 05:40:31PM -0700, Andrew Barnert via Python-ideas wrote:

> If someone wants this proposal, it’s because they believe it’s _not_ a 
> misuse to use a tuple as a frozen list (or a list as a mutable tuple).
> 
> If someone doesn’t want this proposal, the most likely reason 
> (although admittedly there are others) is because they believe it _is_ 
> a misuse to use a tuple as a frozen list.

I don't think it is necessary to believe that it is *always* misuse, but 
only that it is *often* misuse and therefore `==` ought to take the 
conservative position and refuse to guess.

I expect that nearly every Python programmer of sufficient experience 
has used a tuple as a de facto "frozen list" because it works and 
practicality beats purity. But that doesn't mean that I want my 
namedtuple PlayerStats(STR=10, DEX=12, INT=13, CON=9, WIS=8, CHR=12) to 
compare equal to my list [10, 12, 13, 9, 8, 12] by default.


> It’s not always a misuse; it’s sometimes perfectly idiomatic to use a 
> tuple as an immutable hashable sequence. It doesn’t just happen to 
> 'work', it works, for principled reasons (tuple is a Sequence), and 
> this is a good thing.[1]
> 
> It’s just that it’s _also_ common (probably a lot more common, but 
> even that isn’t necessary) to use it as an anonymous struct.
> 
> So, the OP is right that (1,2,3)==[1,2,3] would sometimes be handy, 
> the opponents are right that it would often be misleading, and the 
> question isn’t which one is right, it’s just how often is often. And 
> the answer is obviously: often enough that it can’t be ignored. And 
> that’s all that matters here.

Yes, I think there's a genuine need here.


> And that’s why tuple is different from frozenset. Very few uses of 
> frozenset are as something other than a frozen set, so it’s almost 
> never misleading that frozensets equal sets; plenty of tuples aren’t 
> frozen lists, so it would often be misleading if tuples equaled lists.


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 11:39 PM Steven D'Aprano  wrote:

> "More important" according to whose needs?
>

I dunno.  To mine? To "beginner programmers"? To numeric computation?

I can weaken my 'note' to 'purport' if that helps.

(3) Whereas the proposed duck-typing sequence equality relies on
> the ordinary meaning of equality, applied element by element,
> ignoring the type of the containers.
>

I think this one is our main disagreement.  I think a meaning for
"equality" in which a tuple is equal (equivalent) to a list with the same
items inside it is strikingly different from the ordinary meaning of
equality.  I don't deny that it is sometimes a useful question to ask, but
it is a new and different question than the one answered by '==' currently.

In my mind, this new kind of equality is MORE DIFFERENT from the current
meaning than would be case-folded equivalence of strings, for example.

> (1) They require a specialised equivalence relation apart from
> `==`. Such as math.isclose(), a case insensitive comparison,
> a JSON comparison.
>

Actually, this could perfectly well live on the types rather than in the
modules.  I mean, I could do it today by defining .__eq__() on some
subclasses of strings, floats, dicts, etc. if I wanted to.

But hypothetically (I'm not proposing this), we could also define new
operators .__eq2__(), .__eq3__(), etc. that would be called when Python
programmers used the operators `===`, ``, etc.  With these new
operators in hand, we might give meanings to these new kinds of equivalence:

(1, 2, 3) === [1, 2, 3]   # has_same_items()
"David" === "daviD"   # a.upper() == b.upper()
"David"  "dabit"  # soundex(a, b)
3.14159265 === 3.14159266  # math.isclose(a, b)

It's competely general in a way that the other equivalences aren't.
>

Umm... no, it's really not.  It's a special kind of equivalence that I
guess applies to the Sequence ABC.  Or maybe the Collection ABC? But to be
really useful, it probably needs to work with things that don't register
those ABCs themselves.  I would surly expect:

(1, 2, 3) === np.array([1, 2, 3])

Also, if this were a thing.  But what about dicts, which are now ordered,
and hence sequence-like? Or dict.keys() if not the dict itself?

I'm sure reasonable answers could be decided for questions like that, but
this is FAR from "completely general" or a transparent extension of current
equality.

-- 
The dead increasingly dominate and strangle both the living and the
not-yet born.  Vampiric capital and undead corporate persons abuse
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become abortifacients against new conceptions.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Andrew Barnert via Python-ideas

On May 8, 2020, at 20:36, Dan Sommers <2qdxy4rzwzuui...@potatochowder.com> 
wrote:
> 
> On Fri, 8 May 2020 17:40:31 -0700
> Andrew Barnert via Python-ideas  wrote:
> 
>> So, the OP is right that (1,2,3)==[1,2,3] would sometimes be handy,
>> the opponents are right that it would often be misleading, and the
>> question isn’t which one is right ...
> 
> That's a good summary.  Thank you.  :-)
> 
>> [1] If anyone still wants to argue that using a tuple as a hashable
>> sequence instead of an anonymous struct is wrong, how would you change
>> this excerpt of code:
>> 
>>memomean = memoize(mean, key=tuple)
>>def player_stats(player):
>># …
>>… = memomean(player.scores) …
>># …
>> 
>> Player.scores is a list of ints, and a new one is appended after each
>> match, so a list is clearly the right thing. But you can’t use a list
>> as a cache key. You need a hashable sequence of the same values. And
>> the way to spell that in Python is tuple.
> 
> Very clever.  

I don’t think it’s particularly clever. And that’s fine—using common idioms 
usually is one of the least clever ways to do something out of the infinite 
number of possible ways. Because being intuitively the one obvious way tends to 
be important to becoming an idiom, and it tends to run counter to being clever. 
(Being concise, using well-tested code, and being efficient are also often 
important, but being clever doesn’t automatically give you any of those.)

> Then again, it wouldn't be python-ideas if it were that
> simple!  "hashable sequence of the same values" is too strict.  I think
> all memoize needs is a key function such that if x != y, then key(x) !=
> key(y).

Well, it does have to be hashable. (Unless you’re proposing to also replace the 
dict with an alist or something?) I suppose it only needs to be a hashable 
_encoding_ of a sequence of the same values, but surely the simplest encoding 
of a sequence is the sequence itself, so, unless “hashable sequence” is 
impossible (which it obviously isn’t), who cares?

>def key(scores):
>','.join(str(-score * 42) for score in scores)

This is still a sequence. If you really want to get clever, why not:

def key(scores):
return sum(prime**score for prime, score in zip(calcprimes(), scores))

But this just demonstrates why you don’t really want to get clever. It’s more 
code to write, read, and debug than tuple, easier to get wrong, harder to 
understand, and almost certainly slower, and the only advantage is that it 
deliberately avoids meeting a requirement that we technically didn’t need but 
got for free.

> Oh, wait, even that's too strict.  All memoize really needs is if
> mean(x) != mean(y), then key(x) != key(y):
> 
>memomean = memoize(mean, key=mean)
>def player_stats(player):
># …
>… = memomean(player.scores) …
># …

Well, it seems pretty unlikely that calculating the mean to use it as a cache 
key will be more efficient than just calculating the mean, but hey, if you’ve 
got benchmarks, benchmarks always win. :)

(In fact, I predicted that memoizing here would be a waste of time in the first 
place, because the only players likely to have equal score lists to earlier 
players would be the ones with really short lists—but someone wanted to try it 
anyway, and he was able to show that it did speed up the script on our test 
data set by something like 10%. Not nearly as much as he’d hoped, but still 
enough that it was hard to argue against keeping it.)
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 04:51:04PM +0100, Henk-Jaap Wagenaar wrote:
> FYI, it does show in my version on gmail and on the mailman version.
> 
> 
> BTW, I think strings do showcase some problems with this idea, .EQ. (as
> defined by Steven) is not recursive, which I think will be
> unworkable/unhelpful:

The sample implementation I gave was explicitly described as "very 
roughly". In no way was it intended to be the reference implementation. 
It was intended to begin the process of deciding on the semantics, not 
end it.


> ((0, 1), (1, 2)) and ([0, 1], [1, 2]) are not equal under the new operator
> (or new behaviour of == depending as per the OP) which I think goes
> completely against the idea in my book.

(1) Ahmed has already accepted that changing `==` is not suitable, so 
please let us all stop beating that dead horse! `==` is not going to 
change.

(2) The intention is for 

((0, 1), [1, 2], 'ab') .EQ. [[0, 1], (1, 2), ['a', 'b']]

to return True, as well as similar examples. On the other hand, this is 
not just a "flattening equality" operator, this would return False:

((0, 1), (1, 2)) .EQ. ((0,), (1, 2, 3))

since (0, 1) and (0,) have different lengths.


> If it were (replace x==y with x == y || x .EQ. y with appropriate error
> handling), strings would not work as expected (I would say), e.g.:
> 
> [["f"], "o", "o"] .EQ. "foo"
> 
> because a an element of a string is also a string.

Why would that not work?

* ["f"] .EQ. "f" is true since they both have length 1 and their 
  zeroth elements are equal;
* "o" .EQ. "o" is true;
* "o" .EQ. "o" is still true the second time :-)
* so the whole thing is true.


> Worse though, I guess
> any equal length string that are not equal:
> 
> "foo" .EQ. "bar"
> 
> would crash as it would keep recursing (i.e. string would have to be
> special cased).

Yes. Is that a problem? As I already pointed out, it will also need to 
handle cycles. For example:

a = [1, 2]
a.append(a)
b = (1, 2, [1, 2, a])

and I would expect that a .EQ. b should be True:

len(a) == len(b)
a[0] == b[0]  # 1 == 1
a[1] == b[1]  # 2 == 2
a[2] == b[2]  # a == a

so that's perfectly well-defined.


> What I do sometimes use/want (more often for casual coding/debugging, not
> real coding) is something that compares two objects created from JSON/can
> be made into JSON whether they are the same, sometimes wanting to ignore
> certain fields or tell you what the difference is.

Feel free to propose that as a separate issue.



-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 03:12:10PM -0400, Dan Sommers wrote:
> On Sat, 9 May 2020 03:39:53 +1000
> Steven D'Aprano  wrote:
> 
> > This proposal is a narrow one: its the same as list or tuple equality, 
> > but duck-typed so that the container type doesn't matter.
> 
> Okay.  Good.
> 
> "Container," however, is a dangerous word in this context.  According to
> https://docs.python.org/3/library/stdtypes.html, lots of things are
> "conrtainers."  Can they all be sequence-equal to each other?

All(?) sequences are containers, but not all containers are sequences, 
so no.

> Of particular note might be sets, which don't have an inherent order.  I
> am in no way proposing that sequence-equal be extended to cover sets,
> which by definition can't really be a sequence.

This is a very good question, thank you. I think that this ought to 
exclude mappings and sets, at least initially. Better to err on the side 
of caution than to overshoot by adding too much and then being stuck 
with it.

The primary use-case here is for sequences. Comparisons between sets and 
sequences are certainly possible, but one has to decide on a 
case-by-case basis what you mean. For example, are these equal?

{1, 2} and (1, 1, 2)

I don't know and I don't want to guess, so leave it out.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 07:24:43PM -0400, David Mertz wrote:
> On Fri, May 8, 2020, 6:39 PM Steven D'Aprano
> 
> > So what? Why is this relevant? This is not a proposal for a generalised
> > equivalence relation. If you want one of those feel free to propose a
> > competing idea.
> >
> 
> The OP, with a certain degree of support from you, is asking for changing
> the meaning of an operator to enshrine one particular equivalence relation
> as syntactically blessed by the language.

https://mail.python.org/archives/list/python-ideas@python.org/message/IRIOEXRYRZYYRHLUYGUOGHZJO6NZBEJD/

Ahmed is no longer asking for any change to the `==` operator. That's 
multiple dozens of emails out of date.

> I note that that equivalence relation is not more important than numerous
> other equivalence relations

"More important" according to whose needs?

I would agree with you that a string method to do case insensitive 
comparisons would be very useful. I would certainly use that instead of

a.casefold() == b.casefold()

especially if there was an opportunity to make it more efficient and 
avoid copying of two potentially very large strings.

But why is that relevant? There is no conflict or competition between a 
new string method and new operator. We could have both! "Case 
insensitive string comparisons would be useful" is an argument for case 
insensitive string comparisons, it's not an argument against an 
unrelated proposal.

> and hence should simply be a function
> returning a Boolean answer.

Sure, we can always write a function.

But for something as fundamental as a type of equality, there is much to 
be said for an operator. That's why we have operators in the first 
place, including `==` itself, rather than using the functions from the 
operator module.

> I'm certain you understand this, I'm not sure why the facade otherwise.

Façade, "a showy misrepresentation intended to conceal something 
unpleasant" (WordNet). Synonyms include deception, fakery, false front, 
fraud, imposture, insincerity, simulacrum, subterfuge, and trick.

I'm sorry to hear that you are *certain* of my state of mind, and 
even sorrier that you believe I am lying, but I assure you, I truly do 
believe that these other equivalence relations are not relevant.

And here is why:

(1) They require a specialised equivalence relation apart from 
`==`. Such as math.isclose(), a case insensitive comparison, 
a JSON comparison.

(2) As such they ought to go into their specialist namespaces:

- case-insensitive string comparisons should be a string method, 
  or at worst, a function in the string module;

- a JSON-comparison probably should go into the json module;

- fuzzy numeric equality should probably go into the math module
  (and that's precisely where isclose() currently exists).

And hence they are not in competition with this proposal.

(3) Whereas the proposed duck-typing sequence equality relies on 
the ordinary meaning of equality, applied element by element, 
ignoring the type of the containers.

We can think of this as precisely the same as list equality, or tuple 
equality, minus the initial typecheck that both operands are lists. If 
you can understand list equality, you can understand this. You don't 
have to ask "what counts as close enough? what's JSON?" etc. It's just 
the regular sequence equality but with ducktyping on containers.

It's competely general in a way that the other equivalences aren't.

If you think that these other proposals are worth having, and are more 
useful, then *make the proposal* and see if you get interest from other 
people.

You said that you would prefer to have a JSON-comparing comparison 
operator. If you use a lot of JSON, I guess that might be useful. Okay, 
make the case for that to be an operator! I'm listening. I might be 
convinced. You might get that operator in 3.10, and Python will be a 
better language. Just start a new, competing, proposal for it.

But if you're not prepared to make that case, then don't use the 
existence of something you have no intention of ever asking for as a 
reason to deny something which others do want.

"Python doesn't have this hammer, therefore you shouldn't get this 
screwdriver" is a non-sequitor and a lousy argument.

> If
> you think that yes, that has_same_items() really is that much more
> important, present the case for that rather than irrelevant pedantics.

That's what I'm trying to do.

> But '==' does not guarantee either symmetry or transitivity either. Not
> even among objects that intense to mean it in more-or-less the ordinary
> sense.

Is this intended as an argument for or against this proposal, or is it 
another "irrelevant pedantics" you just accused me of making?

In any case, it is an exaggerated position to take. Among ints, or 
strings, or floats excluding NANs, `==` holds with all the usual 
properties we expect:

* x == x for all ints, strings and floats excluding NANs;
* if, and only if, x == y, then y ==

[Python-ideas] Re: Equality between some of the indexed collections

On Fri, 8 May 2020 17:40:31 -0700
Andrew Barnert via Python-ideas  wrote:

> So, the OP is right that (1,2,3)==[1,2,3] would sometimes be handy,
> the opponents are right that it would often be misleading, and the
> question isn’t which one is right ...

That's a good summary.  Thank you.  :-)

> [1] If anyone still wants to argue that using a tuple as a hashable
> sequence instead of an anonymous struct is wrong, how would you change
> this excerpt of code:
> 
> memomean = memoize(mean, key=tuple)
> def player_stats(player):
> # …
> … = memomean(player.scores) …
> # …
> 
> Player.scores is a list of ints, and a new one is appended after each
> match, so a list is clearly the right thing. But you can’t use a list
> as a cache key. You need a hashable sequence of the same values. And
> the way to spell that in Python is tuple.

Very clever.  Then again, it wouldn't be python-ideas if it were that
simple!  "hashable sequence of the same values" is too strict.  I think
all memoize needs is a key function such that if x != y, then key(x) !=
key(y).

def key(scores):
','.join(str(-score * 42) for score in scores)

memomean = memoize(mean, key=key)
def player_stats(player):
# …
… = memomean(player.scores) …
# …

Oh, wait, even that's too strict.  All memoize really needs is if
mean(x) != mean(y), then key(x) != key(y):

memomean = memoize(mean, key=mean)
def player_stats(player):
# …
… = memomean(player.scores) …
# …

But we won't go there.  ;-)
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Andrew Barnert via Python-ideas

On May 6, 2020, at 05:22, Richard Damon wrote:
>
> In my mind, tuples and lists seem very different concepts, that just
> happen to work similarly at a low level (and because of that, are
> sometimes 'misused' as each other because it happens to 'work').

I think this thread has gotten off track, and this is really the key issue here.

If someone wants this proposal, it’s because they believe it’s _not_ a misuse
to use a tuple as a frozen list (or a list as a mutable tuple).

If someone doesn’t want this proposal, the most likely reason (although
admittedly there are others) is because they believe it _is_ a misuse to use a
tuple as a frozen list.

It’s not always a misuse; it’s sometimes perfectly idiomatic to use a tuple as
an immutable hashable sequence. It doesn’t just happen to 'work', it works, for
principled reasons (tuple is a Sequence), and this is a good thing.[1]

It’s just that it’s _also_ common (probably a lot more common, but even that
isn’t necessary) to use it as an anonymous struct.

So, the OP is right that (1,2,3)==[1,2,3] would sometimes be handy, the
opponents are right that it would often be misleading, and the question isn’t
which one is right, it’s just how often is often. And the answer is obviously:
often enough that it can’t be ignored. And that’s all that matters here.

And that’s why tuple is different from frozenset. Very few uses of frozenset
are as something other than a frozen set, so it’s almost never misleading that
frozensets equal sets; plenty of tuples aren’t frozen lists, so it would often
be misleading if tuples equaled lists.

—-

[1] If anyone still wants to argue that using a tuple as a hashable sequence
instead of an anonymous struct is wrong, how would you change this excerpt of
code:

memomean = memoize(mean, key=tuple)
def player_stats(player):
# …
… = memomean(player.scores) …
# …

Player.scores is a list of ints, and a new one is appended after each match, so
a list is clearly the right thing. But you can’t use a list as a cache key. You
need a hashable sequence of the same values. And the way to spell that in
Python is tuple.

And that’s not a design flaw in Python, it’s a feature. (Shimmer is a floor wax
_and_ a dessert topping!) Sure, when you see a tuple, the default first guess
is that it’s an anonymous struct—but when it isn’t, it’s usually so obvious
from context that you don’t even have to think about it. It’s confusing a lot
less often than, say, str, and it’s helpful a lot more often.
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020, 6:39 PM Steven D'Aprano

> So what? Why is this relevant? This is not a proposal for a generalised
> equivalence relation. If you want one of those feel free to propose a
> competing idea.
>

The OP, with a certain degree of support from you, is asking for changing
the meaning of an operator to enshrine one particular equivalence relation
as syntactically blessed by the language.

I note that that equivalence relation is not more important than numerous
other equivalence relations, and hence should simply be a function
returning a Boolean answer.

I'm certain you understand this, I'm not sure why the facade otherwise. If
you think that yes, that has_same_items() really is that much more
important, present the case for that rather than irrelevant pedantics.

(To be pedantic: at least the allclose() one is not an equivalence
> relation, as it would be possible to have
>
> isclose(a, b) and isclose(b, c) but not isclose(a, c). But that's a
> by-the-by.)
>

Yes. And moreover, we can have:

   numpy.isclose(a, b) != numpy.isclose(b, a)

The math module had a different approach that guarantees symmetry. Neither
is a bug, they are just different.

But '==' does not guarantee either symmetry or transitivity either. Not
even among objects that intense to mean it in more-or-less the ordinary
sense.

Practicality beats purity. If .isclose() calls things equivalent, them for
most purposes the calculation will be fine if you substitute. A strict
mathematical equivalence relation is more... Well, strict. But in terms of
what programmers usually care about, this is fluff.

Fwiw, my proposal is "just write a simple function." I've made that
proposal several times in this thread... But I don't think it's exactly PEP
in nature.
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 03:16:48PM -0400, David Mertz wrote:
> On Fri, May 8, 2020 at 1:47 PM Steven D'Aprano  wrote:
> 
> > All of which are red herrings that are completely off-topic for this
> > proposal. This proposal has nothing to do with:
> >
> > > case_insensitive_eq(a, b)
> > > same_json_representation(a, b)
> > > allclose(a, b)  # A version of this is in NumPy
> > > nan_ignoring_equality(a, b)
> >
> 
> I think you are trying very hard to miss the point.  Yes... all of those
> functions that express a kind of equivalence are different from the OP
> proposal.  But ALL OF THEM have just as much claim to being called
> equivalence as the proposal does.

So what? Why is this relevant? This is not a proposal for a generalised 
equivalence relation. If you want one of those feel free to propose a 
competing idea.

(To be pedantic: at least the allclose() one is not an equivalence 
relation, as it would be possible to have 

isclose(a, b) and isclose(b, c)

but not isclose(a, c). But that's a by-the-by.)

Duck-typed sequence-equality requires no specialised equivalence 
relation. It's exactly the same as existing notions of container 
equality, except without the type-check on the container. It is a 
generic operation, not a specialised one like checking for fuzzy numeric 
close-enoughness, or JSON representations.

If you want case insensitive string equality, propose a new string method.

> If we could only extend the '=='
> operator to include one other comparison, I would not choose the OP's
> suggestion over those others.  Similarly, if '===' or '.EQ.' could only
> have one meaning, the OP proposal would not be what I would most want.

Great! Start your own proposal in a new thread then and stop hijacking 
this one.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Chris Angelico

On Sat, May 9, 2020 at 4:43 AM Dan Sommers
<2qdxy4rzwzuui...@potatochowder.com> wrote:
>
> On Sat, 9 May 2020 03:01:15 +1000
> Steven D'Aprano  wrote:
>
> > On Thu, May 07, 2020 at 11:04:16AM -0400, Dan Sommers wrote:
> > > On Thu, 7 May 2020 21:18:16 +1000
> > > Steven D'Aprano  wrote:
> > >
> > > > > The strongest equality is the "is" operator
> > > >
> > > > Please don't encourage the conceptual error of thinking of `is` as
> > > > *equality*, not even a kind of equality. It doesn't check for equality,
> > > > it checks for *identity* and we know that there is at least one object
> > > > in Python where identical objects aren't equal:
> > > >
> > > > py> from math import nan
> > > > py> nan is nan
> > > > True
> > > > py> nan == nan
> > > > False
> > >
> > > We'd better agree to disagree on this one.
> >
> > Why? In what way is there any room for disagreement at all?
>
> I believe that the "is" operator is a test for some kind of equality,
> and you apparently don't.
>
> > * some equal objects are not identical;
> > * and some identical objects are not equal.
> >
> > It is a matter of fact that in Python `is` tests for object identity,
> > not equality:
> >
> > https://docs.python.org/3/reference/expressions.html#is-not
>
> Section 6.10 is entitled Comparisons, and lists both "is" and "==" as
> comparison operators.
>
> I admit that my use of the word "strongest" (to describe the "is"
> operator) and my conceptual ordering of different kinds of equality
> fails in the light of NaNs.  Curse you, IEEE Floating Point!  :-)
>
> Then again, that same documentation states "User-defined classes that
> customize their comparison behavior should follow some consistency
> rules, if possible."  One of the consistency rules is "Equality
> comparison should be reflexive. In other words, identical objects should
> compare equal," and that rule is summarized as "x is y implies x == y."
>
> So I'm not the only one who thinks of "is" as a kind of equality.  :-)
>

The documentation doesn't say that "is" represents equality, but only
that, in general, an object should be equal to itself. Identity is
still a completely separate concept to equality.

There's a concept of "container equality" that is expressed as "x is y
or x == y", but that's still a form of equality check. "x is y" on its
own is not an equality check. It's an identity check. Obviously it's a
comparison, but so are many other things :)

ChrisA
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, 9 May 2020 03:39:53 +1000
Steven D'Aprano  wrote:

> This proposal is a narrow one: its the same as list or tuple equality, 
> but duck-typed so that the container type doesn't matter.

Okay.  Good.

"Container," however, is a dangerous word in this context.  According to
https://docs.python.org/3/library/stdtypes.html, lots of things are
"conrtainers."  Can they all be sequence-equal to each other?

Of particular note might be sets, which don't have an inherent order.  I
am in no way proposing that sequence-equal be extended to cover sets,
which by definition can't really be a sequence.
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 1:47 PM Steven D'Aprano  wrote:

> All of which are red herrings that are completely off-topic for this
> proposal. This proposal has nothing to do with:
>
> > case_insensitive_eq(a, b)
> > same_json_representation(a, b)
> > allclose(a, b)  # A version of this is in NumPy
> > nan_ignoring_equality(a, b)
>

I think you are trying very hard to miss the point.  Yes... all of those
functions that express a kind of equivalence are different from the OP
proposal.  But ALL OF THEM have just as much claim to being called
equivalence as the proposal does.  If we could only extend the '=='
operator to include one other comparison, I would not choose the OP's
suggestion over those others.  Similarly, if '===' or '.EQ.' could only
have one meaning, the OP proposal would not be what I would most want.

Which is NOT, of course, to say that I don't think
`containers_with_same_contents()` isn't a reasonable function.  But it's
just that, a function.

-- 
The dead increasingly dominate and strangle both the living and the
not-yet born.  Vampiric capital and undead corporate persons abuse
the lives and control the thoughts of homo faber. Ideas, once born,
become abortifacients against new conceptions.
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, 9 May 2020 04:29:46 +1000
Steven D'Aprano  wrote:

> On Fri, May 08, 2020 at 07:52:10PM +0200, Alex Hall wrote:
> 
> > Would the proposal come with a new magic dunder method which can be
> > overridden, or would it be like `is`?
> 
> An excellent question! I don't think there needs to be a dunder. Calling 
> this "sequence-equal":
> 
> Two sequences are "sequence-equal" if:
> 
> - they have the same length;
> 
> - for each pair of corresponding elements, the two elements are 
>   either equal, or sequence-equal.

FWIW, the "or sequence-equal" part is what makes sequences of sequences
"recursively equivalent."  Without that, [(1, 2)] would not be
sequence-equal to ([1, 2]).
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Chris Angelico

On Sat, May 9, 2020 at 4:17 AM Alex Hall  wrote:
>
> On Fri, May 8, 2020 at 7:52 PM David Mertz  wrote:
>>>
>>> Me: For non-singleton immutables, identity is not really a meaningful 
>>> thing.  I mean, other than in a debugger or code profiler, or something 
>>> special like that. I honestly do not know whether, e.g. '(1, "a", 3.5) is 
>>> (1, "a", 3.5)'.  I'll go try it, but I won't be sure the answer for every 
>>> implementation, version, and even runtime, whether that answer will be 
>>> consistent.
>>
>>
>> So I did try it.  I did not necessarily expect these particular results.  
>> Moreover, I have a hunch that with PyPy JIT, something similar might 
>> actually give different answers at different points when the same line was 
>> encountered in a running interpreter.  Not this example, but something else 
>> that might cache values only later.
>>
>> I haven't done anything sneaky with the version at those paths.  They are 
>> all what the environment name hints they should be.  PyPy is at 3.6, which 
>> is the latest version on conda-forge.
>>
>> 810-tmp % $HOME/miniconda3/envs/py2.7/bin/python -c 'print((1, "a", 3.5) is 
>> (1, "a", 3.5))'
>> False
>> 811-tmp % $HOME/miniconda3/envs/py3.4/bin/python -c 'print((1, "a", 3.5) is 
>> (1, "a", 3.5))'
>> False
>> 812-tmp % $HOME/miniconda3/envs/py3.8/bin/python -c 'print((1, "a", 3.5) is 
>> (1, "a", 3.5))'
>> :1: SyntaxWarning: "is" with a literal. Did you mean "=="?
>> True
>> 813-tmp % $HOME/miniconda3/envs/pypy/bin/python -c 'print((1, "a", 3.5) is 
>> (1, "a", 3.5))'
>> True
>> 814-tmp % $HOME/miniconda3/envs/py1/bin/python -c 'print (1, "a", 3.5) is 
>> (1, "a", 3.5)'
>> 0
>
>
> This is because of the peephole optimiser, right?
>
> ```
> Python 3.8.0 (default, Oct 30 2019, 12:16:01)
> [GCC 7.4.0] on linux
> Type "help", "copyright", "credits" or "license" for more information.
> >>> (1, "a", 3.5) is (1, "a", 3.5)
> :1: SyntaxWarning: "is" with a literal. Did you mean "=="?
> True
> >>> x = (1, "a", 3.5)
> >>> x == (1, "a", 3.5)
> True
> >>> x is (1, "a", 3.5)
> :1: SyntaxWarning: "is" with a literal. Did you mean "=="?
> False
> >>>
> ```

I think you're more seeing the module compilation optimizations here.
Inside a single compilation unit (usually a module), constants will
often be shared. So, for instance:

>>> exec("""
... x = "Hello, world!"
... y = "Hello, world!"
... print(x is y)
... """)
True

But if you do those lines individually at the REPL, you'll get False.
Of course, a compliant Python interpreter is free to either collapse
them or keep them separate, but this optimization helps to keep .pyc
file sizes down, for instance.

ChrisA
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, 9 May 2020 03:01:15 +1000
Steven D'Aprano  wrote:

> On Thu, May 07, 2020 at 11:04:16AM -0400, Dan Sommers wrote:
> > On Thu, 7 May 2020 21:18:16 +1000
> > Steven D'Aprano  wrote:
> > 
> > > > The strongest equality is the "is" operator
> > > 
> > > Please don't encourage the conceptual error of thinking of `is` as 
> > > *equality*, not even a kind of equality. It doesn't check for equality, 
> > > it checks for *identity* and we know that there is at least one object 
> > > in Python where identical objects aren't equal:
> > > 
> > > py> from math import nan
> > > py> nan is nan
> > > True
> > > py> nan == nan
> > > False
> > 
> > We'd better agree to disagree on this one.
> 
> Why? In what way is there any room for disagreement at all?

I believe that the "is" operator is a test for some kind of equality,
and you apparently don't.

> * some equal objects are not identical;
> * and some identical objects are not equal.
> 
> It is a matter of fact that in Python `is` tests for object identity, 
> not equality:
> 
> https://docs.python.org/3/reference/expressions.html#is-not

Section 6.10 is entitled Comparisons, and lists both "is" and "==" as
comparison operators.

I admit that my use of the word "strongest" (to describe the "is"
operator) and my conceptual ordering of different kinds of equality
fails in the light of NaNs.  Curse you, IEEE Floating Point!  :-)

Then again, that same documentation states "User-defined classes that
customize their comparison behavior should follow some consistency
rules, if possible."  One of the consistency rules is "Equality
comparison should be reflexive. In other words, identical objects should
compare equal," and that rule is summarized as "x is y implies x == y."

So I'm not the only one who thinks of "is" as a kind of equality.  :-)

> You don't even need to look at such exotic objects as NANs to see that 
> `is` does not test for equality. None of these will return True:
> 
> [] is []
> 1.5 is Fraction(3, 2)
> (a := {}) is a.copy()
> 
> even though the operands are clearly equal.

The OP wants [1, 2, 3] == (1, 2, 3) to return True, even though the
operands are clearly not equal.

> > YAGNI is how I feel about an operator that compares sequences element by
> > element.
> 
> Remember that list-to-list and tuple-to-tuple already perform the same 
> sequence element-by-element comparison.

My mistake.  I should have said "... compares arbitrary sequences of
varying types ..." and not just "sequences."

> All this proposal adds is *duck-typing* to the comparison, for when 
> it doesn't matter what the container type is, you care only about the 
> values in the container. Why be forced to do a possibly expensive (and 
> maybe very expensive!) manual coercion to a common type just to check 
> the values for equality element by element, and then throw away the 
> coerced object?

Then I'll write a function that iterates over both sequences and
compares the pairs of elements.  There's no need to coerce one or both
completes sequences.

> If you have ever written `a == list(b)` or similar, then You Already
> Needed It :-)

I don't recall having written that.  I do end up writing 'a == set(b)'
when a is a set and b is a list, rather than building b as a set in the
first place, but sets aren't sequences.

> > FWIW, I agree:  list != tuple.  When's the last time anyone asked for
> > the next element of a tuple?
> 
> Any time you have written:
> 
> for obj in (a, b, c): ...
> 
> you are asking for the next element of a tuple.

I have been known to write:

for x in a, b, c:

(without the parenthesis), usually in the REPL, but only because it's
convenient and it works.  In other programming languages that don't
allow iteration over tuples, I use lists instead.
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 07:52:10PM +0200, Alex Hall wrote:

> Would the proposal come with a new magic dunder method which can be
> overridden, or would it be like `is`?

An excellent question! I don't think there needs to be a dunder. Calling 
this "sequence-equal":

Two sequences are "sequence-equal" if:

- they have the same length;

- for each pair of corresponding elements, the two elements are 
  either equal, or sequence-equal.

The implementation may need to check for cycles (as ordinary equality 
does). It may also shortcut some equality tests by doing identity tests, 
as ordinary container equality does.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 01:26:05PM -0400, David Mertz wrote:

> The distinction you make seems both pedantic and factually wrong.

Which distinction are you referring to? The one between `is` and `==`?

And in what way is it factually wrong?

> More flat-footed still is "equal objects are ones whose .__eq__() 
> method returns something truthy."

Nevertheless, flat-footed or not, that is broadly the only meaning 
of equality that has any meaning in Python. Two objects are equal if, 
and only if, the `==` operator returns true when comparing them. That's 
what equality means in Python!

(There are a few nuances and complexities to that when it comes to 
containers, which may short-cut equality tests with identity tests for 
speed.)

> It doesn't actually need to define any of the behaviors we think of as 
> equality/equivalence.

Indeed. Which is why we cannot require any of those behaviours for the 
concept of equality in Python.

> Both '==' and 'is' are ways of saying equivalent-for-a-purpose.

`==` is the way to say "equal", where equal means whatever the class 
wants it to mean. If you want to describe that as "equivalent-for-a- 
purpose", okay.

But `is` compares exactly and only "object identity", just as the docs 
say, just as the implementation, um, implements. That's not an 
equivalence, at least not in the plain English sense of the word, 
because an equivalence implies at least the possibility of *distinct* 
objects being equivalent:

a is equivalent to b but a is not identical to b

Otherwise why use the term "equivalent" when you actually mean "is the 
same object"? By definition you cannot have:

a is identical to b but a is not identical to b

so in this sense `is` is not a form of equivalence, it is just *is*.

The mathematical sense of an equivalence relation is different: object 
identity certainly is an equivalence relation.

[...]
> Given that different Python
> implementations will give different answers for 'some_int is
> some_other_int' where they are "equal" in an ordinary sense, identity isn't
> anything that special in most cases.

Right. 

Remind me -- why are we talking about identity? Is it relevant to the 
proposal for a duck-typing container equals operator?

[...]
> The only cases where identity REALLY has semantics I would want to rely on
> are singletons like None and True, and I guess for custom mutable objects
> when you want to make sure which state is separated versus shared. Well,
> OK, I guess lists are an example of that already for the same reason.

So... only None, and True and False, and other singletons like 
NotImplemented, and custom mutable objects, and builtin mutable objects 
like list and dict and set, and typically for classes, functions and 
modules unless you're doing something weird.

Okay.

> For non-singleton immutables, identity is not really a meaningful thing.

It's of little practical use except to satisfy the caller's curiousity 
about implementation details.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 7:52 PM David Mertz  wrote:

> Me: For non-singleton immutables, identity is not really a meaningful
>> thing.  I mean, other than in a debugger or code profiler, or something
>> special like that. I honestly do not know whether, e.g. '(1, "a", 3.5) is
>> (1, "a", 3.5)'.  I'll go try it, but I won't be sure the answer for every
>> implementation, version, and even runtime, whether that answer will be
>> consistent.
>>
>
> So I did try it.  I did not necessarily expect these particular results.
> Moreover, I have a hunch that with PyPy JIT, something similar might
> actually give different answers at different points when the same line was
> encountered in a running interpreter.  Not this example, but something else
> that might cache values only later.
>
> I haven't done anything sneaky with the version at those paths.  They are
> all what the environment name hints they should be.  PyPy is at 3.6, which
> is the latest version on conda-forge.
>
> 810-tmp % $HOME/miniconda3/envs/py2.7/bin/python -c 'print((1, "a", 3.5)
> is (1, "a", 3.5))'
> False
> 811-tmp % $HOME/miniconda3/envs/py3.4/bin/python -c 'print((1, "a", 3.5)
> is (1, "a", 3.5))'
> False
> 812-tmp % $HOME/miniconda3/envs/py3.8/bin/python -c 'print((1, "a", 3.5)
> is (1, "a", 3.5))'
> :1: SyntaxWarning: "is" with a literal. Did you mean "=="?
> True
> 813-tmp % $HOME/miniconda3/envs/pypy/bin/python -c 'print((1, "a", 3.5) is
> (1, "a", 3.5))'
> True
> 814-tmp % $HOME/miniconda3/envs/py1/bin/python -c 'print (1, "a", 3.5) is
> (1, "a", 3.5)'
> 0
>

This is because of the peephole optimiser, right?

```
Python 3.8.0 (default, Oct 30 2019, 12:16:01)
[GCC 7.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> (1, "a", 3.5) is (1, "a", 3.5)
:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
True
>>> x = (1, "a", 3.5)
>>> x == (1, "a", 3.5)
True
>>> x is (1, "a", 3.5)
:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
False
>>>
```
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Ethan Furman


On 05/08/2020 09:36 AM, Alex Hall wrote:

On Fri, May 8, 2020 at 5:51 PM Henk-Jaap Wagenaar wrote:



FYI, it does show in my version on gmail and on the mailman version. 



Weird, did Ethan's client cut it out?


Ah, no.  I thought you were replying to the code quote above the .EQ. one.  The 
.EQ. quote was not white-space separated from the text around it and I missed 
it.

--
~Ethan~
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 7:48 PM Steven D'Aprano  wrote:

> On Fri, May 08, 2020 at 01:00:48PM -0400, David Mertz wrote:
>
> > All the discussion following Steven's hypothetical .EQ. operator (yes,
> not
> > a possible spelling) just seems to drive home to me that what everyone
> > wants is simply a function.
> >
> > Many different notions of "equivalence for a particular purpose" have
> been
> > mentioned.
>
> All of which are red herrings that are completely off-topic for this
> proposal. This proposal has nothing to do with:
>
> > case_insensitive_eq(a, b)
> > same_json_representation(a, b)
> > allclose(a, b)  # A version of this is in NumPy
> > nan_ignoring_equality(a, b)
>
> and the only reason I deleted the "recursively equivalent" one is
> because I don't know what it's supposed to mean.
>
> This proposal is a narrow one: its the same as list or tuple equality,
> but duck-typed so that the container type doesn't matter.
>
> Do lists and tuples do case-insensitive comparisons? No. Then neither
> does this proposal.
>
> Do lists and tuples do JSON-repr comparisons? No. Then neither does
> this.
>
> Do lists and tuples do numeric "within some epsilon" isclose comparisons
> (e.g. APL fuzzy equality)? Or ignore NANs? No to both of those. Then
> neither does this proposal.
>

Would the proposal come with a new magic dunder method which can be
overridden, or would it be like `is`?
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[Python-ideas] Re: Equality between some of the indexed collections

>
> Me: For non-singleton immutables, identity is not really a meaningful
> thing.  I mean, other than in a debugger or code profiler, or something
> special like that. I honestly do not know whether, e.g. '(1, "a", 3.5) is
> (1, "a", 3.5)'.  I'll go try it, but I won't be sure the answer for every
> implementation, version, and even runtime, whether that answer will be
> consistent.
>

So I did try it.  I did not necessarily expect these particular results.
Moreover, I have a hunch that with PyPy JIT, something similar might
actually give different answers at different points when the same line was
encountered in a running interpreter.  Not this example, but something else
that might cache values only later.

I haven't done anything sneaky with the version at those paths.  They are
all what the environment name hints they should be.  PyPy is at 3.6, which
is the latest version on conda-forge.

810-tmp % $HOME/miniconda3/envs/py2.7/bin/python -c 'print((1, "a", 3.5) is
(1, "a", 3.5))'
False
811-tmp % $HOME/miniconda3/envs/py3.4/bin/python -c 'print((1, "a", 3.5) is
(1, "a", 3.5))'
False
812-tmp % $HOME/miniconda3/envs/py3.8/bin/python -c 'print((1, "a", 3.5) is
(1, "a", 3.5))'
:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
True
813-tmp % $HOME/miniconda3/envs/pypy/bin/python -c 'print((1, "a", 3.5) is
(1, "a", 3.5))'
True
814-tmp % $HOME/miniconda3/envs/py1/bin/python -c 'print (1, "a", 3.5) is
(1, "a", 3.5)'
0

-- 
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 08, 2020 at 01:00:48PM -0400, David Mertz wrote:

> All the discussion following Steven's hypothetical .EQ. operator (yes, not
> a possible spelling) just seems to drive home to me that what everyone
> wants is simply a function.
> 
> Many different notions of "equivalence for a particular purpose" have been
> mentioned.

All of which are red herrings that are completely off-topic for this 
proposal. This proposal has nothing to do with:

> case_insensitive_eq(a, b)
> same_json_representation(a, b)
> allclose(a, b)  # A version of this is in NumPy
> nan_ignoring_equality(a, b)

and the only reason I deleted the "recursively equivalent" one is 
because I don't know what it's supposed to mean.

This proposal is a narrow one: its the same as list or tuple equality, 
but duck-typed so that the container type doesn't matter.

Do lists and tuples do case-insensitive comparisons? No. Then neither 
does this proposal.

Do lists and tuples do JSON-repr comparisons? No. Then neither does 
this.

Do lists and tuples do numeric "within some epsilon" isclose comparisons 
(e.g. APL fuzzy equality)? Or ignore NANs? No to both of those. Then 
neither does this proposal.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 1:06 PM Steven D'Aprano  wrote:

> Whatever we might feel about equality and identity in the wider
> philosophical sense, in the *Python programming sense* the semantic
> meaning of the two operators are orthogonal:
>
> * some equal objects are not identical;
> * and some identical objects are not equal.
>

You yourself introduced—speculatively—the idea of another equality
operator, .EQ., that would be "equal in some sense not captured by '=='.  I
just posted another comment where I gave function names for six plausibly
useful concepts of "equality" ... well, technically,
equivalence-for-purpose.

The distinction you make seems both pedantic and factually wrong.  More
flat-footed still is "equal objects are ones whose .__eq__() method returns
something truthy."  It doesn't actually need to define any of the behaviors
we think of as equality/equivalence.  I was going to write a silly example
of e.g. throwing a random() into the operation, but I don't think I have to
for the point to be obvious.

Both '==' and 'is' are ways of saying equivalent-for-a-purpose.  For that
matter, so is math.isclose() or numpy.allclose().  Or those json-diff
libraries someone just linked to.  Given that different Python
implementations will give different answers for 'some_int is
some_other_int' where they are "equal" in an ordinary sense, identity isn't
anything that special in most cases.  Strings are likewise sometimes cached
(but differently by version and implementation).

The only cases where identity REALLY has semantics I would want to rely on
are singletons like None and True, and I guess for custom mutable objects
when you want to make sure which state is separated versus shared. Well,
OK, I guess lists are an example of that already for the same reason.

For non-singleton immutables, identity is not really a meaningful thing.  I
mean, other than in a debugger or code profiler, or something special like
that. I honestly do not know whether, e.g. '(1, "a", 3.5) is (1, "a",
3.5)'.  I'll go try it, but I won't be sure the answer for every
implementation, version, and even runtime, whether that answer will be
consistent.

-- 
The dead increasingly dominate and strangle both the living and the
not-yet born.  Vampiric capital and undead corporate persons abuse
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become abortifacients against new conceptions.
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 11:04:16AM -0400, Dan Sommers wrote:
> On Thu, 7 May 2020 21:18:16 +1000
> Steven D'Aprano  wrote:
> 
> > > The strongest equality is the "is" operator
> > 
> > Please don't encourage the conceptual error of thinking of `is` as 
> > *equality*, not even a kind of equality. It doesn't check for equality, 
> > it checks for *identity* and we know that there is at least one object 
> > in Python where identical objects aren't equal:
> > 
> > py> from math import nan
> > py> nan is nan
> > True
> > py> nan == nan
> > False
> 
> We'd better agree to disagree on this one.

Why? In what way is there any room for disagreement at all?

This isn't a matter of subjective opinion, like what's the best Star 
Wars film or whether pineapple belongs on pizza. This is a matter of 
objective fact, like whether Python strings are Unicode or not.

Whatever we might feel about equality and identity in the wider 
philosophical sense, in the *Python programming sense* the semantic 
meaning of the two operators are orthogonal:

* some equal objects are not identical;
* and some identical objects are not equal.

It is a matter of fact that in Python `is` tests for object identity, 
not equality:

https://docs.python.org/3/reference/expressions.html#is-not

If you wish to agree with Bertrand Meyer that reflexivity of equality is 
one of the pillars of civilization:

https://bertrandmeyer.com/2010/02/06/reflexivity-and-other-pillars-of-civilization/

and therefore Python gets equality wrong, you are welcome to that 
opinion, but whether we like it or not equality in Python is not 
necessarily reflexive and as a consequence objects may be identical 
(i.e. the same object) but not equal. Float and Decimal NANs are the 
most obvious examples.

You don't even need to look at such exotic objects as NANs to see that 
`is` does not test for equality. None of these will return True:

[] is []
1.5 is Fraction(3, 2)
(a := {}) is a.copy()

even though the operands are clearly equal.

[...]
> YAGNI is how I feel about an operator that compares sequences element by
> element.

Remember that list-to-list and tuple-to-tuple already perform the same 
sequence element-by-element comparison.

All this proposal adds is *duck-typing* to the comparison, for when 
it doesn't matter what the container type is, you care only about the 
values in the container. Why be forced to do a possibly expensive (and 
maybe very expensive!) manual coercion to a common type just to check 
the values for equality element by element, and then throw away the 
coerced object?

If you have ever written `a == list(b)` or similar, then You Already 
Needed It :-)

> People can write their own functions.  :-)  Or add your .EQ.
> function to the standard library (or even to builtins, and no, I don't
> have a good name).

True, but there are distinct advantages to operators over functions for 
some operations. See Guido's essay:

https://neopythonic.blogspot.com/2019/03/why-operators-are-useful.html

> > It is only that wanting to compare two ordered containers for equality
> > of their items without regard to the type of container is a reasonably
> > common and useful thing to do.
> 
> > Even if we don't want list==tuple to return True -- and I don't! -- we
> > surely can recognise that sometimes we don't care about the
> > container's type, only it's elements.
> 
> Do "reasonably common," "useful," and "sometimes" meet the bar for a new
> operator?  (That's an honest question and not a sharp stick.)

It depends on how common and useful, and how easy it is to find a good 
operator. It might be a brilliant idea stymied by lack of a good 
operator.

We might be forced to use a function because there are no good operators 
left any more, and nobody wants Python to turn into Perl or APL.

> FWIW, I agree:  list != tuple.  When's the last time anyone asked for
> the next element of a tuple?

Any time you have written:

for obj in (a, b, c): ...

you are asking for the next element of a tuple.

A sample from a test suite I just happen to have open at the moment:

# self.isprime_functions is a tuple of functions to test
for func in self.isprime_functions:

for a in (3, 5, 6):
self.assertFalse(sqrt_exists(a, 7))
for a in (2, 6, 7, 8, 10):
self.assertFalse(sqrt_exists(a, 11))

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

All the discussion following Steven's hypothetical .EQ. operator (yes, not
a possible spelling) just seems to drive home to me that what everyone
wants is simply a function.

Many different notions of "equivalence for a particular purpose" have been
mentioned.  We're not going to get a dozen different equality operators
(even Lisp or Javascript don't go that far).  But function names are
plentiful.  So just write your own:

has_same_elements(a, b)
case_insensitive_eq(a, b)
same_json_representation(a, b)
allclose(a, b)  # A version of this is in NumPy
recursively_equivalent(a, b)
nan_ignoring_equality(a, b)

And whatever others you like.  All of these seem straightforwardly relevant
to their particular use case (as do many others not listed).  But none of
them have a special enough status to co-opt the '==' operator or deserve
their own special operator.

-- 
The dead increasingly dominate and strangle both the living and the
not-yet born.  Vampiric capital and undead corporate persons abuse
the lives and control the thoughts of homo faber. Ideas, once born,
become abortifacients against new conceptions.
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 5:51 PM Henk-Jaap Wagenaar <
wagenaarhenkj...@gmail.com> wrote:

> FYI, it does show in my version on gmail and on the mailman version.
> 
>

Weird, did Ethan's client cut it out?


> BTW, I think strings do showcase some problems with this idea, .EQ. (as
> defined by Steven) is not recursive, which I think will be
> unworkable/unhelpful:
>
> ((0, 1), (1, 2)) and ([0, 1], [1, 2]) are not equal under the new operator
> (or new behaviour of == depending as per the OP) which I think goes
> completely against the idea in my book.
>

If we redefined == so that `(0, 1) == [0, 1]`, then it would follow that
`((0, 1), (1, 2)) == ([0, 1], [1, 2])`.

Similarly if `(0, 1) .EQ. [0, 1]`, then it would follow that `((0, 1), (1,
2)) .EQ. ([0, 1], [1, 2])`.


> If it were (replace x==y with x == y || x .EQ. y with appropriate error
> handling), strings would not work as expected (I would say), e.g.:
>
> [["f"], "o", "o"] .EQ. "foo"
>
> because a an element of a string is also a string. Worse though, I guess
> any equal length string that are not equal:
>
> "foo" .EQ. "bar"
>
> would crash as it would keep recursing (i.e. string would have to be
> special cased).
>

Yes, strings would have to be special cased. In my opinion this is another
sign that strings shouldn't be iterable, see the recent heated discussion
at
https://mail.python.org/archives/list/python-ideas@python.org/thread/WKEFHT4JYCL2PMZ5LB6HJRLVP3OGZI56/


> What I do sometimes use/want (more often for casual coding/debugging, not
> real coding) is something that compares two objects created from JSON/can
> be made into JSON whether they are the same, sometimes wanting to ignore
> certain fields or tell you what the difference is. I do not think that
> could ever be an operator, but having a function that can help these kind
> of recursive comparisons would be great (I guess pytest uses/has such a
> function because it pretty nicely displays differences in sets,
> dictionaries and lists which are compared to each others in asserts).
>

Something like https://github.com/fzumstein/jsondiff or
https://pypi.org/project/json-diff/?
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Henk-Jaap Wagenaar

FYI, it does show in my version on gmail and on the mailman version.

BTW, I think strings do showcase some problems with this idea, .EQ. (as
defined by Steven) is not recursive, which I think will be
unworkable/unhelpful:

((0, 1), (1, 2)) and ([0, 1], [1, 2]) are not equal under the new operator
(or new behaviour of == depending as per the OP) which I think goes
completely against the idea in my book.

If it were (replace x==y with x == y || x .EQ. y with appropriate error
handling), strings would not work as expected (I would say), e.g.:

[["f"], "o", "o"] .EQ. "foo"

because a an element of a string is also a string. Worse though, I guess
any equal length string that are not equal:

"foo" .EQ. "bar"

would crash as it would keep recursing (i.e. string would have to be
special cased).

What I do sometimes use/want (more often for casual coding/debugging, not
real coding) is something that compares two objects created from JSON/can
be made into JSON whether they are the same, sometimes wanting to ignore
certain fields or tell you what the difference is. I do not think that
could ever be an operator, but having a function that can help these kind
of recursive comparisons would be great (I guess pytest uses/has such a
function because it pretty nicely displays differences in sets,
dictionaries and lists which are compared to each others in asserts).

On Fri, 8 May 2020 at 16:23, Alex Hall  wrote:

> On Fri, May 8, 2020 at 5:11 PM Ethan Furman  wrote:
>
>> On 05/08/2020 07:50 AM, Alex Hall wrote:
>> > On Fri, May 8, 2020 at 4:46 PM Henk-Jaap Wagenaar wrote:
>> >> On Fri, 8 May 2020 at 14:16, Steven D'Aprano > > wrote:
>> >>
>> >>> If you have ever written something like any of these:
>> >>>
>> >>>  all(x==y for x,y in zip(a, b))
>> >>
>> >> That looks like a zip call that could do with checking its input or
>> strict=True!
>> >
>> > Steven mentioned that originally:
>> >>
>> >> (Aside: if we go down this track, this could be a justification for
>> >> zip_strict to be a builtin; see the current thread(s) on having a
>> >> version of zip which strictly requires its input to be equal length.)
>> >
>> > But since you probably want these expressions to evaluate to false
>> rather than raise an exception when the lengths are different, a strict zip
>> is not appropriate.
>>
>> But if:
>>
>>  short_sequence == long_sequence[:len(short_sequence)]
>>
>> then you'll get True.
>>
>
> So you'd need to just check the lengths first. That was in Steven's older
> code snippet, which I tried to quote and shows in my sent messages but not
> now. Really hating this email quoting.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Jonathan Fine

Here's an example you might want to consider:

>>> from collections import namedtuple

>>> Point = namedtuple('Point', ['x', 'y'])
>>> Point(1, 2)
Point(x=1, y=2)
>>> Point(1, 2) == (1, 2)
True

>>> Polar = namedtuple('Polar', ['r', 'theta'])
>>> Polar(1, 2)
Polar(r=1, theta=2)
>>> Polar(1, 2) == (1, 2)
True

>>> Point(1, 2) == Polar(1, 2)
True
>>> hash(Point(1, 2)) == hash(Polar(1, 2)) == hash((1, 2))
   True

-- 
Jonathan
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 5:11 PM Ethan Furman  wrote:

> On 05/08/2020 07:50 AM, Alex Hall wrote:
> > On Fri, May 8, 2020 at 4:46 PM Henk-Jaap Wagenaar wrote:
> >> On Fri, 8 May 2020 at 14:16, Steven D'Aprano  > wrote:
> >>
> >>> If you have ever written something like any of these:
> >>>
> >>>  all(x==y for x,y in zip(a, b))
> >>
> >> That looks like a zip call that could do with checking its input or
> strict=True!
> >
> > Steven mentioned that originally:
> >>
> >> (Aside: if we go down this track, this could be a justification for
> >> zip_strict to be a builtin; see the current thread(s) on having a
> >> version of zip which strictly requires its input to be equal length.)
> >
> > But since you probably want these expressions to evaluate to false
> rather than raise an exception when the lengths are different, a strict zip
> is not appropriate.
>
> But if:
>
>  short_sequence == long_sequence[:len(short_sequence)]
>
> then you'll get True.
>

So you'd need to just check the lengths first. That was in Steven's older
code snippet, which I tried to quote and shows in my sent messages but not
now. Really hating this email quoting.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Ethan Furman


On 05/08/2020 07:50 AM, Alex Hall wrote:

On Fri, May 8, 2020 at 4:46 PM Henk-Jaap Wagenaar wrote:

On Fri, 8 May 2020 at 14:16, Steven D'Aprano mailto:st...@pearwood.info>> wrote:


If you have ever written something like any of these:

     all(x==y for x,y in zip(a, b))


That looks like a zip call that could do with checking its input or strict=True! 


Steven mentioned that originally:


(Aside: if we go down this track, this could be a justification for
zip_strict to be a builtin; see the current thread(s) on having a
version of zip which strictly requires its input to be equal length.) 


But since you probably want these expressions to evaluate to false rather than 
raise an exception when the lengths are different, a strict zip is not 
appropriate.


But if:

short_sequence == long_sequence[:len(short_sequence)]

then you'll get True.

--
~Ethan~
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, May 8, 2020 at 4:46 PM Henk-Jaap Wagenaar <
wagenaarhenkj...@gmail.com> wrote:

> On Fri, 8 May 2020 at 14:16, Steven D'Aprano  wrote:
>
>> If you have ever written something like any of these:
>>
>> list(a) == list(b)
>> tuple(a) == b
>> ''.join(chars) == mystring
>> all(x==y for x,y in zip(a, b))
>>
>
> That looks like a zip call that could do with checking its input or
> strict=True!
>

Steven mentioned that originally:

We could define this .EQ. operate as *sequence equality*, defined very
> roughly as:
> def .EQ. (a, b):
> return len(a) == len(b) and all(x==y for x, y in zip(a, b))
> (Aside: if we go down this track, this could be a justification for
> zip_strict to be a builtin; see the current thread(s) on having a
> version of zip which strictly requires its input to be equal length.)

But since you probably want these expressions to evaluate to false rather
than raise an exception when the lengths are different, a strict zip is not
appropriate.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-08 Thread Henk-Jaap Wagenaar

On Fri, 8 May 2020 at 14:16, Steven D'Aprano  wrote:

> On Thu, May 07, 2020 at 03:43:23PM +0200, Dominik Vilsmeier wrote:
>
> > >We could define this .EQ. operate as *sequence equality*, defined very
> > >roughly as:
> > >
> > > def .EQ. (a, b):
> > > return len(a) == len(b) and all(x==y for x, y in zip(a, b))
> > >
> > But why do we even need a new operator when this simple function does
> > the job (at least for sized iterables)?
>
> Maybe it doesn't need to be an operator, but operators do have a big
> advantage over functions:
>
> http://neopythonic.blogspot.com/2019/03/why-operators-are-useful.html
>
> On the other hand we only have a limited number of short symbols
> available in ASCII, and using words as operators reduces that benefit.
>
>
> > How common is it to compare two objects where you cannot determine
> > whether one or the other is a tuple or a list already from the
> > surrounding context? In the end these objects must come from somewhere
> > and usually functions declare either list or tuple as their return type.
>
> Never, because we can always determine whether something is a list or
> tuple by inspecting it with type() or isinstance(). But that's missing
> the point! I don't care and don't want to know if it is a tuple or
> list, I only care if it quacks like a sequence of some kind.
>
> The use-case for this is for when you want to compare elements without
> regard to the type of the container they are in. This is a duck-typing
> sequence element-by-element equality test.
>
> If you have ever written something like any of these:
>
> list(a) == list(b)
> tuple(a) == b
> ''.join(chars) == mystring
> all(x==y for x,y in zip(a, b))
>

That looks like a zip call that could do with checking its input or
strict=True!


> then this proposed operator might be just what you need.
>
>
>
> --
> Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Fri, 8 May 2020 23:10:05 +1000
Steven D'Aprano  wrote:

> On Thu, May 07, 2020 at 03:43:23PM +0200, Dominik Vilsmeier wrote:

> > How common is it to compare two objects where you cannot determine
> > whether one or the other is a tuple or a list already from the
> > surrounding context? In the end these objects must come from somewhere
> > and usually functions declare either list or tuple as their return type.

> Never, because we can always determine whether something is a list or 
> tuple by inspecting it with type() or isinstance(). But that's missing 
> the point! I don't care and don't want to know if it is a tuple or 
> list, I only care if it quacks like a sequence of some kind.

> The use-case for this is for when you want to compare elements without 
> regard to the type of the container they are in. This is a duck-typing 
> sequence element-by-element equality test.

To rephrase Dominik's question slighly, how often do you have a block of
code with two sequences of unknown origin?  Sure, I can *hypothisize*
f(x, y) where x and y don't have to be anything more specific than
sequences.  But unless I'm actually writing .EQ., there's some code
inside f that builds x or y, or calls some other function to obtain x or
y, and then I know at least one of the types.

You often ask for real world code that would be simpler or easier to
read or maintain if such-and-such feature existed.  The OP never posted
any such thing; do you have any specific code in mind?

> If you have ever written something like any of these:
> 
> list(a) == list(b)
> tuple(a) == b
> ''.join(chars) == mystring
> all(x==y for x,y in zip(a, b))
> 
> then this proposed operator might be just what you need.

Ever?  Maybe.  Do I need help from a new operator or the standard
library when it comes up?  No, not really.  And except for that
join/mystring example, I find all of thse examples incredibly obvious
and simple to read (although looking up a new function or operator isn't
onerous, and I often learn things when that happens).

-- 
“Atoms are not things.” – Werner Heisenberg
Dan Sommers, http://www.tombstonezero.net/dan
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 03:43:23PM +0200, Dominik Vilsmeier wrote:

> >We could define this .EQ. operate as *sequence equality*, defined very
> >roughly as:
> >
> > def .EQ. (a, b):
> > return len(a) == len(b) and all(x==y for x, y in zip(a, b))
> >
> But why do we even need a new operator when this simple function does
> the job (at least for sized iterables)?

Maybe it doesn't need to be an operator, but operators do have a big 
advantage over functions:

http://neopythonic.blogspot.com/2019/03/why-operators-are-useful.html

On the other hand we only have a limited number of short symbols 
available in ASCII, and using words as operators reduces that benefit.

> How common is it to compare two objects where you cannot determine
> whether one or the other is a tuple or a list already from the
> surrounding context? In the end these objects must come from somewhere
> and usually functions declare either list or tuple as their return type.

Never, because we can always determine whether something is a list or 
tuple by inspecting it with type() or isinstance(). But that's missing 
the point! I don't care and don't want to know if it is a tuple or 
list, I only care if it quacks like a sequence of some kind.

The use-case for this is for when you want to compare elements without 
regard to the type of the container they are in. This is a duck-typing 
sequence element-by-element equality test.

If you have ever written something like any of these:

list(a) == list(b)
tuple(a) == b
''.join(chars) == mystring
all(x==y for x,y in zip(a, b))

then this proposed operator might be just what you need.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 04:42:22PM +0100, Rhodri James wrote:
> On 07/05/2020 10:11, Steven D'Aprano wrote:

> The biggest argument against a second "equals" operator, however it is 
> spelt, is confusion.  Which of these two operators do I want to use for 
> this subtly different question of equality?  Even where we have quite 
> distinct concepts like "==" and "is", people still get muddled.  If we 
> have "==" and "=OMG=" or whatever, that would just be an accident 
> waiting to happen.

I don't think so. The confusion with `is` is particularly acute 
for at least two reasons:

- in regular English it can be a synonym for equals, as in "one and 
  one is two, two and two is four";
- it seems to work sometimes: `1 + 1 is 2` will probably succeed.

If the operator was named differently, we probably wouldn't have many 
people writing `1 + 1 idem 2` or `1 + 1 dasselbe 2` when they wanted 
equality.

I doubt many people would be confused whether they wanted, let's say, 
the `==` operator or the `same_items` operator, especially if `1 + 1 
same_items 2` raised a TypeError.


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Rhodri James


On 07/05/2020 10:11, Steven D'Aprano wrote:

On Sat, May 02, 2020 at 05:12:58AM -, Ahmed Amr wrote:


Currently, when comparing a list of items to an array of the same
items for equality (==) it returns False, I'm thinking that it would
make sense to return True in that context, as we're comparing item
values and we have the same way of indexing both collections, so we
can compare item values.


I'm going to throw out a wild idea (actually not that wild :-) that I'm
sure people will hate for reasons I shall mention afterwards.

Perhaps we ought to add a second "equals" operator?


The biggest argument against a second "equals" operator, however it is 
spelt, is confusion.  Which of these two operators do I want to use for 
this subtly different question of equality?  Even where we have quite 
distinct concepts like "==" and "is", people still get muddled.  If we 
have "==" and "=OMG=" or whatever, that would just be an accident 
waiting to happen.


Cheers,
Rhodri

--
Rhodri James *-* Kynesim Ltd
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Dan Sommers

On Thu, 7 May 2020 21:18:16 +1000
Steven D'Aprano  wrote:

> > The strongest equality is the "is" operator
> 
> Please don't encourage the conceptual error of thinking of `is` as 
> *equality*, not even a kind of equality. It doesn't check for equality, 
> it checks for *identity* and we know that there is at least one object 
> in Python where identical objects aren't equal:
> 
> py> from math import nan
> py> nan is nan
> True
> py> nan == nan
> False

We'd better agree to disagree on this one.

> > The very far ends of that scale are glossing over American
> > vs. British spellings (are "color" and "colour" in some sense equal?),
> 
> YAGNI.

> The proposal here is quite simple and straightforward, there is no
> need to over-generalise it to the infinite variety of possible
> equivalencies than someone might want. People can write their own
> functions.

YAGNI is how I feel about an operator that compares sequences element by
element.  People can write their own functions.  :-)  Or add your .EQ.
function to the standard library (or even to builtins, and no, I don't
have a good name).

> It is only that wanting to compare two ordered containers for equality
> of their items without regard to the type of container is a reasonably
> common and useful thing to do.

> Even if we don't want list==tuple to return True -- and I don't! -- we
> surely can recognise that sometimes we don't care about the
> container's type, only it's elements.

Do "reasonably common," "useful," and "sometimes" meet the bar for a new
operator?  (That's an honest question and not a sharp stick.)

FWIW, I agree:  list != tuple.  When's the last time anyone asked for
the next element of a tuple?  (Okay, if your N-tuple represents a point
in N-space, then you might iterate over the coordinates in order to
discover a bounding box.)

Dan

-- 
“Atoms are not things.” – Werner Heisenberg
Dan Sommers, http://www.tombstonezero.net/dan
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Oscar Benjamin

On Thu, 7 May 2020 at 08:54, Greg Ewing  wrote:
>
> On 7/05/20 1:07 pm, David Mertz wrote:
> > For *most* functions, the substitution
> > principle is fine in Python.  A whole lot of the time, numeric functions
> > can take either an int or a float that are equal to each other and
> > produce results that are equal to each other.
>
> It's not much use for deciding whether two things *should* be
> equal, though, because whatever your opinion on the matter,
> you can come up with a set of functions that satisfy it and
> then say "those are the kinds of functions I mean".
>
> Also, as a definition of equality it seems somewhat circular,
> since if you're not sure whether x == y, you may be equally
> uncertain whether f(x) == f(y) for some f, x, y.

It's not so much a definition of equality as a consistency
requirement. The contrapositive can be very clear: if you already know
that f(x) and f(y) do different things or return unequal objects then
the question of whether x == y is answered.

It's important though that it's not just about equality of return
types: when you carry the principle over from maths to programming
then you need to consider non-pure functions, IO, exceptions being
raised etc.

In simple situations it is nice to be able to duck-type over lists and
tuples but in practice it has to be done carefully by sticking to the
sequence or iterable interfaces precisely or by coercing to a known
type at the entry points of your code. Once you have a large codebase
with lots of objects flying around internally and you no longer know
whether anything is a list or a tuple (or a set...) any more it's just
a mess.

Oscar
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Dominik Vilsmeier


On 07.05.20 11:11, Steven D'Aprano wrote:


On Sat, May 02, 2020 at 05:12:58AM -, Ahmed Amr wrote:


Currently, when comparing a list of items to an array of the same
items for equality (==) it returns False, I'm thinking that it would
make sense to return True in that context, as we're comparing item
values and we have the same way of indexing both collections, so we
can compare item values.


Perhaps we ought to add a second "equals" operator? To avoid
bikeshedding over syntax, I'm initially going to use the ancient 1960s
Fortran syntax and spell it `.EQ.`.

[...]

We could define this .EQ. operate as *sequence equality*, defined very
roughly as:

 def .EQ. (a, b):
 return len(a) == len(b) and all(x==y for x, y in zip(a, b))


But why do we even need a new operator when this simple function does
the job (at least for sized iterables)?

How common is it to compare two objects where you cannot determine
whether one or the other is a tuple or a list already from the
surrounding context? In the end these objects must come from somewhere
and usually functions declare either list or tuple as their return type.

Since for custom types you can already define `__eq__` this really comes
down to the builtin types, among which the theoretical equality between
tuple and list has been debated in much detail but is it used in practice?
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 01:17:37PM +0100, Henk-Jaap Wagenaar wrote:

> Why use "." which has clear syntax problems?

[Me]
> > (For the avoidance of doubt, I know that syntax will not work in 
> > Python because it will be ambiguous. That's why I picked it -- it's 
> > syntax that we can all agree won't work, so we can concentrate on 
> > the semantics not the spelling.)


[Henk-Jaap]
> This can already be done in current Python (this was linked to in a
> previous thread about something else) using a generic solution if you
> change the syntax:
> 
> https://pypi.org/project/infix/
> 
> You could write it as |EQ|, ^EQ^, ... and have it in its own Pypi package.

That's a gimmick and a hack. To be sure, it's a cute hack, but not one I 
would ever use in real code.


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Henk-Jaap Wagenaar

Why use "." which has clear syntax problems?

This can already be done in current Python (this was linked to in a
previous thread about something else) using a generic solution if you
change the syntax:

https://pypi.org/project/infix/

You could write it as |EQ|, ^EQ^, ... and have it in its own Pypi package.

Not sure what IDEs think of this package, they probably hate it...

On Thu, 7 May 2020 at 10:18, Steven D'Aprano  wrote:

> On Sat, May 02, 2020 at 05:12:58AM -, Ahmed Amr wrote:
>
> > Currently, when comparing a list of items to an array of the same
> > items for equality (==) it returns False, I'm thinking that it would
> > make sense to return True in that context, as we're comparing item
> > values and we have the same way of indexing both collections, so we
> > can compare item values.
>
> I'm going to throw out a wild idea (actually not that wild :-) that I'm
> sure people will hate for reasons I shall mention afterwards.
>
> Perhaps we ought to add a second "equals" operator? To avoid
> bikeshedding over syntax, I'm initially going to use the ancient 1960s
> Fortran syntax and spell it `.EQ.`.
>
> (For the avoidance of doubt, I know that syntax will not work in Python
> because it will be ambiguous. That's why I picked it -- it's syntax that
> we can all agree won't work, so we can concentrate on the semantics not
> the spelling.)
>
> We could define this .EQ. operate as *sequence equality*, defined very
> roughly as:
>
> def .EQ. (a, b):
> return len(a) == len(b) and all(x==y for x, y in zip(a, b))
>
> (Aside: if we go down this track, this could be a justification for
> zip_strict to be a builtin; see the current thread(s) on having a
> version of zip which strictly requires its input to be equal length.)
>
> The precise details of the operator are not yet clear to me, for
> instance, should it support iterators or just Sized iterables? But at
> the very least, it would support the original request:
>
> [1, 2, 3] .EQ. (1, 2, 3)
> # returns True
>
>
> The obvious operator for this would be `===` but of course that will
> lead to an immediate and visceral reaction "Argghhh, no, Javascript, do
> not want!!!" :-)
>
> Another obvious operator would be a new keyword `eq` but that would
> break any code using that as a variable.
>
> But apart from the minor inconveniences that:
>
> - I don't know what this should do in detail, only vaguely;
> - and I have no idea what syntax it should have
>
> what do people think of this idea?
>
>
> --
> Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 10:44:01PM +1200, Greg Ewing wrote:
> On 7/05/20 9:11 pm, Steven D'Aprano wrote:
> >I'm initially going to use the ancient 1960s
> >Fortran syntax and spell it `.EQ.`.
> >
> >I know that syntax will not work in Python
> >because it will be ambiguous.
> 
> I'm sure the new parser will handle it just fine!

Yes, but the *human readers* won't. You know that people will write 
things like:

spam.EQ.ham

and then nobody will know whether than means "call the .EQ. operator on 
operands spam and ham" or "lookup the ham attribute on the EQ attribute 
of spam" without looking up the parsing rules.

Let's not turn into Ruby:

https://lucumr.pocoo.org/2008/7/1/whitespace-sensitivity/


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Thu, May 07, 2020 at 06:04:13AM -0400, Dan Sommers wrote:
> On Thu, 7 May 2020 19:11:43 +1000
> Steven D'Aprano  wrote:
> 
> > We could define this .EQ. operate as *sequence equality*, defined very
> > roughly as:
> > 
> > def .EQ. (a, b):
> > return len(a) == len(b) and all(x==y for x, y in zip(a, b))
> 
> Equality and its awkward cousin equivalence are slippery slopes.  Just
> sequences?  That (admittedly rough) function returns true for certain
> mapping arguments.

*shrug*

As I point out later in my post, I don't know whether it should be just 
sequences. Maybe it should be any iterable, although checking them for 
equality will necessarily consume them. But *right now* the proposal on 
the table is to support list==tuple comparisons, which this would do.

(For an especially vague definition of "do" :-)

> What about case-insensitive string matching?

That can be a string method, since it only needs to operate on strings.

> The strongest equality is the "is" operator

Please don't encourage the conceptual error of thinking of `is` as 
*equality*, not even a kind of equality. It doesn't check for equality, 
it checks for *identity* and we know that there is at least one object 
in Python where identical objects aren't equal:

py> from math import nan
py> nan is nan
True
py> nan == nan
False

[...]
> The very far ends of that scale are glossing over American
> vs. British spellings (are "color" and "colour" in some sense equal?),

YAGNI.

The proposal here is quite simple and straightforward, there is no need 
to over-generalise it to the infinite variety of possible equivalencies 
than someone might want. People can write their own functions.

It is only that wanting to compare two ordered containers for equality 
of their items without regard to the type of container is a reasonably 
common and useful thing to do.

Even if we don't want list==tuple to return True -- and I don't! -- we 
surely can recognise that sometimes we don't care about the container's 
type, only it's elements.

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Greg Ewing


On 7/05/20 9:11 pm, Steven D'Aprano wrote:

I'm initially going to use the ancient 1960s
Fortran syntax and spell it `.EQ.`.

I know that syntax will not work in Python
because it will be ambiguous.


I'm sure the new parser will handle it just fine!

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Dan Sommers

On Thu, 7 May 2020 19:11:43 +1000
Steven D'Aprano  wrote:

> We could define this .EQ. operate as *sequence equality*, defined very
> roughly as:
> 
> def .EQ. (a, b):
> return len(a) == len(b) and all(x==y for x, y in zip(a, b))

Equality and its awkward cousin equivalence are slippery slopes.  Just
sequences?  That (admittedly rough) function returns true for certain
mapping arguments.

What about case-insensitive string matching?  Is that more common than
comparing (or wanting to compare) arbitrary sequences?  What about an
operator for normalized (in the Unicode sense of the word),
case-insensitive string comparison?  (There's a precedent:  Common
Lisp's equalp function does case-insensitive string matching.)

The strongest equality is the "is" operator, and then the == operator,
and ISTM that you're now extending this idea to another class of
equivalency.  The very far ends of that scale are glossing over American
vs. British spellings (are "color" and "colour" in some sense equal?),
or even considering two functions "the same" if they produce the same
outputs for the same inputs.

One of Python's premises and strengths is strong typing; please don't
start pecking away at that.  Do beginners expect that [1, 2, 3] == (1,
2, 3)?  No.  Do experts expect that [1, 2, 3] == (1, 2, 3)?  No.  So who
does?  Programmers working on certain applications, or with multiple
[pre-existing] libraries, or without a coherent design.

These all seem like appliction level (or even design level) problems, or
maybe a series of dunder methods / protocols to define various levels of
equivalence (the ability of my inbox and my brain to handle the
resulting bikeshedding notwithstanding).  YMMV.

Just my thoughts.

-- 
“Atoms are not things.” – Werner Heisenberg
Dan Sommers, http://www.tombstonezero.net/dan
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Alex Hall

This reminded me of another recent message so I decided to find that and
link it here:
https://mail.python.org/archives/list/python-ideas@python.org/message/7ILSYYOFPQL5DVH5DGIGQSL6PYFYF7PC/

It seemed like a more useful thing to do before I discovered that you wrote
that too...

On Thu, May 7, 2020 at 11:20 AM Steven D'Aprano  wrote:

> On Sat, May 02, 2020 at 05:12:58AM -, Ahmed Amr wrote:
>
> > Currently, when comparing a list of items to an array of the same
> > items for equality (==) it returns False, I'm thinking that it would
> > make sense to return True in that context, as we're comparing item
> > values and we have the same way of indexing both collections, so we
> > can compare item values.
>
> I'm going to throw out a wild idea (actually not that wild :-) that I'm
> sure people will hate for reasons I shall mention afterwards.
>
> Perhaps we ought to add a second "equals" operator? To avoid
> bikeshedding over syntax, I'm initially going to use the ancient 1960s
> Fortran syntax and spell it `.EQ.`.
>
> (For the avoidance of doubt, I know that syntax will not work in Python
> because it will be ambiguous. That's why I picked it -- it's syntax that
> we can all agree won't work, so we can concentrate on the semantics not
> the spelling.)
>
> We could define this .EQ. operate as *sequence equality*, defined very
> roughly as:
>
> def .EQ. (a, b):
> return len(a) == len(b) and all(x==y for x, y in zip(a, b))
>
> (Aside: if we go down this track, this could be a justification for
> zip_strict to be a builtin; see the current thread(s) on having a
> version of zip which strictly requires its input to be equal length.)
>
> The precise details of the operator are not yet clear to me, for
> instance, should it support iterators or just Sized iterables? But at
> the very least, it would support the original request:
>
> [1, 2, 3] .EQ. (1, 2, 3)
> # returns True
>
>
> The obvious operator for this would be `===` but of course that will
> lead to an immediate and visceral reaction "Argghhh, no, Javascript, do
> not want!!!" :-)
>
> Another obvious operator would be a new keyword `eq` but that would
> break any code using that as a variable.
>
> But apart from the minor inconveniences that:
>
> - I don't know what this should do in detail, only vaguely;
> - and I have no idea what syntax it should have
>
> what do people think of this idea?
>
>
> --
> Steven
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, May 02, 2020 at 05:12:58AM -, Ahmed Amr wrote:

> Currently, when comparing a list of items to an array of the same 
> items for equality (==) it returns False, I'm thinking that it would 
> make sense to return True in that context, as we're comparing item 
> values and we have the same way of indexing both collections, so we 
> can compare item values.

I'm going to throw out a wild idea (actually not that wild :-) that I'm 
sure people will hate for reasons I shall mention afterwards.

Perhaps we ought to add a second "equals" operator? To avoid 
bikeshedding over syntax, I'm initially going to use the ancient 1960s 
Fortran syntax and spell it `.EQ.`.

(For the avoidance of doubt, I know that syntax will not work in Python 
because it will be ambiguous. That's why I picked it -- it's syntax that 
we can all agree won't work, so we can concentrate on the semantics not 
the spelling.)

We could define this .EQ. operate as *sequence equality*, defined very 
roughly as:

def .EQ. (a, b):
return len(a) == len(b) and all(x==y for x, y in zip(a, b))

(Aside: if we go down this track, this could be a justification for 
zip_strict to be a builtin; see the current thread(s) on having a 
version of zip which strictly requires its input to be equal length.)

The precise details of the operator are not yet clear to me, for 
instance, should it support iterators or just Sized iterables? But at 
the very least, it would support the original request:

[1, 2, 3] .EQ. (1, 2, 3)
# returns True

The obvious operator for this would be `===` but of course that will 
lead to an immediate and visceral reaction "Argghhh, no, Javascript, do 
not want!!!" :-)

Another obvious operator would be a new keyword `eq` but that would 
break any code using that as a variable.

But apart from the minor inconveniences that:

- I don't know what this should do in detail, only vaguely;
- and I have no idea what syntax it should have

what do people think of this idea?

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Greg Ewing


On 7/05/20 1:07 pm, David Mertz wrote:
For *most* functions, the substitution 
principle is fine in Python.  A whole lot of the time, numeric functions 
can take either an int or a float that are equal to each other and 
produce results that are equal to each other.


It's not much use for deciding whether two things *should* be
equal, though, because whatever your opinion on the matter,
you can come up with a set of functions that satisfy it and
then say "those are the kinds of functions I mean".

Also, as a definition of equality it seems somewhat circular,
since if you're not sure whether x == y, you may be equally
uncertain whether f(x) == f(y) for some f, x, y.

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Alex Hall

On Thu, May 7, 2020 at 2:33 AM Oscar Benjamin 
wrote:

> On Sat, 2 May 2020 at 20:50, Serhiy Storchaka  wrote:
> >
> > 02.05.20 21:34, Ahmed Amr пише:
> > > I see there are ways to compare them item-wise, I'm suggesting to bake
> > > that functionality inside the core implementation of such indexed
> > > structures.
> > > Also those solutions are direct with tuples and lists, but it wouldn't
> > > be as direct with arrays-lists/tuples comparisons for example.
> >
> > If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
> > == hash([1, 2, 3])`.
>
> This is the key point. Much of the other discussion in this thread
> seems to be bogged down in the mathematical interpretation of tuples
> and sequences but if I was to take something from maths here it would
> be the substitution principle of equality:
> https://en.wikipedia.org/wiki/Equality_(mathematics)#Basic_properties
>
> What the substitution principle essentially says is
>if x == y then f(x) == f(y)
> for any function f such that f(x) is well defined.
>
> What that means is that I should be able to substitute x for y in any
> context where x would work without any change of behaviour.

We discussed this and Dominik pointed out that set and frozenset already
break this property, specifically for hash(). Do you think frozensets and
normal sets should never be equal?
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-07 Thread Chris Angelico

On Thu, May 7, 2020 at 12:26 PM Oscar Benjamin
 wrote:
>
> On Thu, 7 May 2020 at 02:07, David Mertz  wrote:
> >
> > That's the point though.  For *most* functions, the substitution principle 
> > is fine in Python.  A whole lot of the time, numeric functions can take 
> > either an int or a float that are equal to each other and produce results 
> > that are equal to each other.  Yes, I can write something that will 
> > sometimes overflow for floats but not ints.  Yes, I can write something 
> > where a rounding error will pop up differently between the types.  But 
> > generally, numeric functions are "mostly the same most of the time" with 
> > float vs. int arguments.
>
> The question is whether you (or Chris) care about calculating things
> accurately with floats or ints. If you do try to write careful code
> that calculates things for one or the other you'll realise that there
> is no way to duck-type anything nontrivial because the algorithms for
> exact vs inexact or bounded vs unbounded arithmetic are very different
> (e.g. sum vs fsum). If you are not so concerned about that then you
> might say that 1 and 1.0 are "acceptably interchangeable".
>

I most certainly DO care about accurate integer calculations, which is
one of the reasons I'm very glad to have separate int and float types
(ahem, ECMAScript, are you eavesdropping here?). In any situation
where I would consider them equivalent, it's actually the float that I
want (it's absolutely okay if I have to explicitly truncate a float to
int if I want to use it in that context), so the only way they'd not
be equivalent is if the number I'm trying to represent actually isn't
representable. Having to explicitly say "n + 0.0" to force it to be a
float isn't going to change that, so there's no reason to make that
explicit.

For the situations where things like fsum are important, it's great to
be able to grab them. For situations where you have an integer number
of seconds and want to say "delay this action by N seconds" and it
wants a float? It should be fine accepting an integer.

> Please understand though that I am not proposing that 1==1.0 should be
> changed. It is supposed to be a simple example of the knock on effect
> of defining __eq__ between non-equivalent objects.

Definitely not. I'm just arguing against your notion that equality
should ONLY be between utterly equivalent things. It's far more useful
to allow more things to be equal.

ChrisA
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread David Mertz

On Wed, May 6, 2020 at 10:26 PM Oscar Benjamin 
wrote:

> > That's the point though.  For *most* functions, the substitution
> principle is fine in Python.  A whole lot of the time, numeric functions
> can take either an int or a float that are equal to each other and produce
> results that are equal to each other.  Yes, I can write something that will
> sometimes overflow for floats but not ints.  Yes, I can write something
> where a rounding error will pop up differently between the types.  But
> generally, numeric functions are "mostly the same most of the time" with
> float vs. int arguments.
>
> The question is whether you (or Chris) care about calculating things
> accurately with floats or ints. If you do try to write careful code
> that calculates things for one or the other you'll realise that there
> is no way to duck-type anything nontrivial because the algorithms for
> exact vs inexact or bounded vs unbounded arithmetic are very different
> (e.g. sum vs fsum).

Sure. But a great many things I calculate are not particularly exact.  If I
want the mean of about a hundred numbers that are each somewhere in the
interval [1, 1e6], I'm probably not very interested in 1 ulp errors in
64-bit floating point.

And when I *do* care about being exact, I can either cast the arguments to
the appropriate type or raise an exception for the unexpected type.  If my
function deals with primes of thousands of digits, int is more
appropriate.  But maybe I want a Decimal of some specific precision.  Or a
Fraction. Or maybe I want to use gmpy as an external type for greater
precision.  If it's just `x = myfavoritetype(x)` as the first line of the
function, that's easy to do.

> I have fixed enough real examples of bugs relating to
> this to come to the conclusion that making non-interchangeable objects
> compare equal with == is an attractive nuisance.

Yeah, sometimes.  But not nearly as much of an attractive nuisance as using
`==` between to floating point numbers rather than math.isclose() or
numpy.isclose().  My students trip over ` (0.1+0.2)+0.3 == 0.1+(0.2+0.3)` a
lot more often than they trip over `1.0 == 1`.

-- 
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not-yet born.  Vampiric capital and undead corporate persons abuse
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Oscar Benjamin

On Thu, 7 May 2020 at 02:07, David Mertz  wrote:
>
> That's the point though.  For *most* functions, the substitution principle is 
> fine in Python.  A whole lot of the time, numeric functions can take either 
> an int or a float that are equal to each other and produce results that are 
> equal to each other.  Yes, I can write something that will sometimes overflow 
> for floats but not ints.  Yes, I can write something where a rounding error 
> will pop up differently between the types.  But generally, numeric functions 
> are "mostly the same most of the time" with float vs. int arguments.

The question is whether you (or Chris) care about calculating things
accurately with floats or ints. If you do try to write careful code
that calculates things for one or the other you'll realise that there
is no way to duck-type anything nontrivial because the algorithms for
exact vs inexact or bounded vs unbounded arithmetic are very different
(e.g. sum vs fsum). If you are not so concerned about that then you
might say that 1 and 1.0 are "acceptably interchangeable".

Please understand though that I am not proposing that 1==1.0 should be
changed. It is supposed to be a simple example of the knock on effect
of defining __eq__ between non-equivalent objects.

> This doesn't say whether tuple is as similar to list as frozenset is to set.  
> But the answer to that isn't going to be answered by examples constructed to 
> deliberately obtain (non-)substitutability for the sake of argument.

Those examples are not for the sake of argument: they are simple
illustrations. I have fixed enough real examples of bugs relating to
this to come to the conclusion that making non-interchangeable objects
compare equal with == is an attractive nuisance. It seems useful when
you play with toy examples in the REPL but isn't actually helpful when
you try to write any serious code.

This comes up particularly often in sympy because:

1. Many contributors strongly feel that A == B should "do the right
thing" (confusing structural and mathematical equality)
2. Many calculations in sympy are cached and the cache can swap A and
B if A == B.
3. There are a lot of algorithms that make heavy use of ==.

The issues are the same elsewhere though: gratuitously making objects
compare equal with == is a bad idea unless you are happy to substitute
one for the other. Otherwise what is the purpose of having them
compare equal in the first place?

Oscar
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread David Mertz

I'm afraid, Oscar, that you seem to have painted yourself into a reductio
ad absurdum.  We need a healthy dose of "practicality beats purity" thrown
in here.

What the substitution principle essentially says is

>if x == y then f(x) == f(y)
> for any function f such that f(x) is well defined.
> I've come to the seemingly obvious conclusion that if there is *any*
> difference between x and y then it's always better to say that x != y.
>

>>> x = 3.1415
>>> y = 3.1415
>>> x == y
True
>>> f = id
>>> f(x) == f(y)
False

I'm very happy to agree that "but id() isn't the kind of function I meant!"

That's the point though.  For *most* functions, the substitution principle
is fine in Python.  A whole lot of the time, numeric functions can take
either an int or a float that are equal to each other and produce results
that are equal to each other.  Yes, I can write something that will
sometimes overflow for floats but not ints.  Yes, I can write something
where a rounding error will pop up differently between the types.  But
generally, numeric functions are "mostly the same most of the time" with
float vs. int arguments.

This doesn't say whether tuple is as similar to list as frozenset is to
set.  But the answer to that isn't going to be answered by examples
constructed to deliberately obtain (non-)substitutability for the sake of
argument.


-- 
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not-yet born.  Vampiric capital and undead corporate persons abuse
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become abortifacients against new conceptions.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Chris Angelico

On Thu, May 7, 2020 at 10:33 AM Oscar Benjamin
 wrote:
>
> I've come to the seemingly obvious conclusion that if there is *any*
> difference between x and y then it's always better to say that x != y.
>

And having worked in languages where floats and integers are
fundamentally different beasts, I disagree: it is extremely practical
(even if not pure) to have them compare equal. SourcePawn (the
language generally used for modding games like Counter-Strike) is
strongly-typed and does not allow floats and ints to be used
interchangeably - except that you can do arithmetic and they'll be
type-folded. So if you have a function TakeDamage that expects a
floating-point amount of damage, and another function GetHealth that
returns the player's health as an integer, you have to add 0.0 to the
integer before it can be used as a float. Actual line of code from one
of my mods:

SDKHooks_TakeDamage(client, inflictor, attacker,
GetClientHealth(client) + 0.0, 0, weapon);

Every language has to choose where it lands on the spectrum of "weak
typing" (everything can be converted implicitly) to "strong typing"
(explicit conversions only), and quite frankly, both extremes are
generally unusable. Python tends toward the stricter side, but with an
idea of "type" that is at times abstract (eg "iterable" which can
cover a wide variety of concrete types); and one of those very
important flexibilities is that numbers that represent the same value
can be used broadly interchangeably. This is a very good thing.

ChrisA
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Oscar Benjamin

On Sat, 2 May 2020 at 20:50, Serhiy Storchaka  wrote:
>
> 02.05.20 21:34, Ahmed Amr пише:
> > I see there are ways to compare them item-wise, I'm suggesting to bake
> > that functionality inside the core implementation of such indexed
> > structures.
> > Also those solutions are direct with tuples and lists, but it wouldn't
> > be as direct with arrays-lists/tuples comparisons for example.
>
> If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
> == hash([1, 2, 3])`.

This is the key point. Much of the other discussion in this thread
seems to be bogged down in the mathematical interpretation of tuples
and sequences but if I was to take something from maths here it would
be the substitution principle of equality:
https://en.wikipedia.org/wiki/Equality_(mathematics)#Basic_properties

What the substitution principle essentially says is
   if x == y then f(x) == f(y)
for any function f such that f(x) is well defined.

What that means is that I should be able to substitute x for y in any
context where x would work without any change of behaviour. We don't
need to do any deep maths to see how that principle can be applied in
Python but if you try to follow it rigorously then you'll see that
there are already counterexamples in the language for example

>>> x = 1000
>>> y = 1000.0
>>> f = lambda z: 100**z
>>> x == y
True
>>> f(x) # fine

>>> f(y) # not fine
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 1, in 
OverflowError: (34, 'Result too large')

Given a list x and a tuple y with equivalent elements x and y will not
be interchangeable because one is not hashable and the other is not
mutable so there are functions where one is usable but the other is
not. Following the same reasoning set/frozenset should not compare
equal.

In SymPy there are many different mathematical objects that people
feel should (on mathematical grounds) compare "equal". This happens
enough that there is a section explaining this in the tutorial:
https://docs.sympy.org/latest/tutorial/gotchas.html#equals-signs
The terms "structural equality" and "mathematical equality" are used
to distinguish the different kinds of equality with == being used for
the structural sense.

For example the sympy expression Pow(2, 2, evaluate=False) gives an
object that looks like 2**2. This does mathematically represent the
number 4 but the expression itself is not literally the number 4 so
the two expressions are mathematically equal but not structurally
equal:

>>> from sympy import Pow
>>> p = Pow(2, 2, evaluate=False)
>>> p
2**2
>>> p.doit()
4
>>> p == 4
False
>>> p.doit() == 4
True

This distinction is important because at the programmatic level p and
4 are not interchangeable. For example p being a Pow has attributes
base and exp that 4 will not have. In sympy most objects are immutable
and hashable and are heavily used in sets and dicts. Following the
substitution principle matters not least because Python has baked the
use of ==/__eq__ into low-level data structures so objects that
compare equal with == will literally be interchanged:

>>> {1, 1.0}
{1}
>>> {1.0, 1}
{1.0}

All the same many sympy contributors have felt the need to define
__eq__ methods that will make objects of different types compare equal
and there are still examples in the sympy codebase. These __eq__
methods *always* lead to bugs down the line though (just a matter of
time).

I've come to the seemingly obvious conclusion that if there is *any*
difference between x and y then it's always better to say that x != y.

Oscar
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Gerrit Holl

On Sat, 2 May 2020 at 15:06, Ahmed Amr  wrote:
>
> I'd like to take your opinion on modifying some of the indexed collections 
> like tuples, lists, arrays to evaluate its equality to True when having the 
> same items in the same indexes.
> Currently, when comparing a list of items to an array of the same items for 
> equality (==)  it returns False, I'm thinking that it would make sense to 
> return True in that context, as we're comparing item values and we have the 
> same way of indexing both collections, so we can compare item values.
>
> So what do you think about applying such behavior on collections that can be 
> indexed the same way such as tuples, lists, and arrays?
>
> Example: (Current)
>
> import array
> tuple_ = (1.1, 2.2, 3.3)
> list_ = [1.1, 2.2, 3.3]
> array_ = array.array('f', [1.1, 2.2, 3.3])
>
> # all of the following prints False.
> print(tuple_ == list_)
> print(tuple_ == array_)
> print(array_ == list_)

>>> import numpy as np
>>> np.array_equal((1.1, 1.2, 1.3), [1.1, 1.2, 1.3])
True
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Richard Damon

On 5/6/20 3:04 AM, Greg Ewing wrote:
> On 6/05/20 1:58 pm, Henk-Jaap Wagenaar wrote:
>> I'd say the difference is just one of semantics and as a
>> mathematician I would consider tuples and sequences as "isomorphic",
>> in fact, the set-theoretical construction of tuples as functions is
>> *identical* to the usual definition of sequences: i.e. they are just
>> two interpretations of the the same object depending on your point of
>> view.
>
> Maybe the small subset of mathematicians that concern themselves
> with trying to define everything in terms of sets, but I don't
> think the majority of mathematicians think like that in their
> everyday work. It's certainly at odds with the way I see tuples
> and sequences being used in mathematics.
>
> As well as the same type vs. different types thing, here are
> some other salient differences:
>
> - Infinite sequences make sense, infinite tuples not so much.
>
> - Sequences are fundamentally ordered, whereas tuples are not
> ordered in the same sense. Any apparent ordering in a tuple is
> an artifact of the way we conventionally write them. If we were
> in the habit of labelling the elements of a tuple and writing
> things like (x:1, y:2, z:3) then we wouldn't have to write them
> in any particular order -- (y:2, x:1, z:3) would be the same
> tuple.
>
In my mind, tuples and lists seem very different concepts, that just
happen to work similarly at a low level (and because of that, are
sometimes 'misused' as each other because it happens to 'work').

To me, tuples are things when the position of the thing very much
matters, you understand the meaning of the Nth element of a tuple
because it IS the Nth element of the tuple. It isn't so important that
the Nth is after the (N-1)th element, so we could define our universe of
tuples in a different order then it might still make sense, but we then
need to reorder ALL the tuples of that type. A coordinate makes a great
example of a tuple, we think of the 1st element of the coordinate as 'X'
due to convention, and in the tuple it gets in meaning from its position
in the tuple.

A list on the other hand is generally not thought of in that way. A list
might not be ordered, or it might be, and maybe there is SOME value in
knowing that an item is the Nth on the list, but if it is an ordered
list, it is generally more meaningful to think of the Nth item in
relation to the (N-1)th and (N+1)th items.

Adding an element to a tuple generally doesn't make sense (unless it is
transforming it to a new type of tuple, like from 2d to 3d), but
generally adding an item to a list does.

This makes their concepts very different. Yes, you might 'freeze' a list
by making it a tuple so it becomes hashable, but then you are really
thinking of it as a 'frozen list' not really a tuple. And there may be
times you make a mutable tuple by using a list, but then you are
thinking of it as a mutable tuple, not a list. And these are exceptional
cases, not the norm.

-- 
Richard Damon
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread jdveiga

Greg Ewing wrote:
> On 6/05/20 7:45 pm, Henk-Jaap Wagenaar wrote:
> > So... say you are solving a problem in 1d, you do
> > that on a real number 
> > x, right? Now you solve it in 2d, so you do your work on a pair (x, y), 
> > then you might solve it in 3d and do your work on a triplet (x, y, z). A 
> > few days later you generalize it to n-dimensions and you get a 
> > sequence
> > At this point I would say that you haven't created an infinite
> tuple, you've created an infinite sequence of finite tuples.
> > Then, a few days later you generalize it to infinite
> > sequences (x_1, 
> > x_2, ...).
> > Now here I would stop and say, wait a minute, what does this
> proof look like? I'm willing to bet it involves things that
> assume some kind of intrinsic order to the elements of this
> "tuple". If it does, and it's an extension to the finite
> dimensional cases, then I would say you were really dealing
> with sequences, not tuples, right from the beginning.
> Now I must admit I was a bit hesitant about writing that
> statement, because in quantum theory, for example, one often
> deals with vector spaces having infinitely many dimensions.
> You could consider an element of such a space as being an
> infinite tuple.
> However, to even talk about such an object, you need to be
> able to write formulas involving the "nth element", and those
> formulas will necessarily depend on the numerical value of
> n. This gives the elements an intrinsic order, and they will
> have relationships to each other that depend on that order.
> This makes the object more like a sequence than a tuple.
> Contrast this with, for example, a tuple (x, y, z) representing
> coordinates in a geometrical space. There is no inherent
> sense in which the x coordinate comes "before" the y coordinate;
> that's just an accident of the order we chose to write them
> down in. We could have chosen any other order, and as long as
> we were consistent about it, everything would still work.
> This, I think, is the essence of the distinction between
> tuples and sequences in mathematics. Elements of sequences
> have an inherent order, whereas elements of a tuple have at
> best an arbitrarily-imposed order.

However, in Python, tuples and lists are both sequences, ordered sets of 
elements.

So it is not completely unreasoned to see them as Ahmed Amr is proposing: that 
is, so similar types that you can expect that if they have the same element, 
they are equal. (Like frozensets and sets in the "set type" domain).

Indeed, tuples and lists are equivalent in Python: `(list() == list(tuple()) 
and tuple(list()) == tuple()) is True`.

Do not misunderstand me. I agree with the idea that tuples and lists are 
different by design while frozenset and sets are not (as Steven D'Aprano 
pointed out in a previous posts).

But considering tuples and lists as just ordered sets of elements and based 
their equality on their elements, not in their type, is an appealing idea. I 
think that some Pythonists would not disagree.

A different thing is the practicality of this.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Steven D'Aprano

On Wed, May 06, 2020 at 07:15:22PM +1200, Greg Ewing wrote:
> On 6/05/20 6:12 pm, Steven D'Aprano wrote:
> 
> >https://mathworld.wolfram.com/n-Tuple.html
> >https://mathworld.wolfram.com/List.html
> >https://mathworld.wolfram.com/Sequence.html
> 
> I get the feeling those definitions are a result of seeing
> things through Mathematica-tinted glasses.

Oh I'm not agreeing with them, I'm just pointing out that the people who 
hang around math.stackexchange and the people who write for Mathworld 
don't agree.

It is difficult to capture all the nuances of common usage in a short 
definition. Based purely on dictionary definitions, 'The Strolling 
Useless' is precisely the same meaning as 'The Walking Dead' but no 
native English speaker would confuse the two, and I'm pretty sure that 
few mathematicians would call the origin of the Cartesian Plane "a 
sequence" even if it does meet the definition perfectly :-)

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Greg Ewing

On 6/05/20 7:45 pm, Henk-Jaap Wagenaar wrote:
So... say you are solving a problem in 1d, you do that on a real number
x, right? Now you solve it in 2d, so you do your work on a pair (x, y),
then you might solve it in 3d and do your work on a triplet (x, y, z). A
few days later you generalize it to n-dimensions and you get a
*sequence*

At this point I would say that you haven't created an infinite
tuple, you've created an infinite sequence of finite tuples.

Then, a few days later you generalize it to infinite sequences (x_1,
x_2, ...).

Now here I would stop and say, wait a minute, what does this
proof look like? I'm willing to bet it involves things that
assume some kind of intrinsic order to the elements of this
"tuple". If it does, and it's an extension to the finite
dimensional cases, then I would say you were really dealing
with sequences, not tuples, right from the beginning.

Now I must admit I was a bit hesitant about writing that
statement, because in quantum theory, for example, one often
deals with vector spaces having infinitely many dimensions.
You could consider an element of such a space as being an
infinite tuple.

However, to even talk about such an object, you need to be
able to write formulas involving the "nth element", and those
formulas will necessarily depend on the numerical value of
n. This gives the elements an intrinsic order, and they will
have relationships to each other that depend on that order.
This makes the object more like a sequence than a tuple.

Contrast this with, for example, a tuple (x, y, z) representing
coordinates in a geometrical space. There is no inherent
sense in which the x coordinate comes "before" the y coordinate;
that's just an accident of the order we chose to write them
down in. We could have chosen any other order, and as long as
we were consistent about it, everything would still work.

This, I think, is the essence of the distinction between
tuples and sequences in mathematics. Elements of sequences
have an inherent order, whereas elements of a tuple have at
best an arbitrarily-imposed order.

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Henk-Jaap Wagenaar

TL;DR: the maths does not matter. Programming language (design)/computer
science/data structures should lead this discussion! Also, -1 on this
proposal, -1000 on having it apply to strings.

Feel free to read on if you want to hear some ramblings of somebody who
does not get to use their academic knowledge of maths enough seeing an
opportunity...

On Wed, 6 May 2020 at 07:18, Steven D'Aprano  wrote:

> On Wed, May 06, 2020 at 02:58:01AM +0100, Henk-Jaap Wagenaar wrote:
>
> > I don't think that is accurate to represent as a representation of "a
> > mathematician". The top voted answer here disagrees:
> >
> https://math.stackexchange.com/questions/122595/whats-the-difference-between-tuples-and-sequences
> >
> > "A sequence requires each element to be of the same type.
> > A tuple can have elements with different types."
>
> Are you saying that you can't have a sequence that alternates between
> ints and rationals, say, or ints and surds (reals)?
>
> The sequence A_n = sqrt(n) from n=0 starts off int, int, real, ... so
> there is that.
>

That's a sequence in the reals (or algebraics or some other set that
contains square roots), of which a subsequence also happens to live in the
integers. A square is still a rectangle.

> For what its worth, Wolfram Mathworld disagrees with both Greg's comment
> and the stackexchange answer, stating that a tuple is just a synonym for
> a list, and that both lists and sequences are ordered sets:
>
> https://mathworld.wolfram.com/n-Tuple.html
>
> https://mathworld.wolfram.com/List.html

These two above pertain to data structures in computer science, not
mathematics. An "ordered set" is not a mathematical term I have every come
across, but if it is, it means exactly as how they define a sequence
(though you would have to extend it to infinite sequences to allow infinite
ordered sets):

>
> https://mathworld.wolfram.com/Sequence.html
>
>
>
The notation ([image: a_1], [image: a_2], ..., [image: a_n])  is the same
as saying it is a sequence in some set X^n (if not given an X, setting X =
{a_1, ..., a_n} works, is that cheating? Yes. Is that a problem in
set-theoretic mathematics? Not in this case anyway)

> > The common usage for both is: you have a tuple of (Z, +) representing the
> > Abelian group of addition (+) on the integers (Z), whereas you have the
> > sequence {1/n}_{n \in N} converging to 0 in the space Q^N (rational
> > infinite sequences) for example.
>
> One can come up with many other usages. I think a far more common use
> for tuples are the ordered pairs used for coordinates:
>
> (1, 2)
>

I would call that an ordered pair, or, a sequence of length 2.

>
> So although tuples are ordered sets, and sequences are ordered sets, the
> way they are used is very different. One would not call the coordinate
> (1, 2) a sequence 1 followed by 2, and one would not normally consider a
> sequence such as [0, 2, 4, 6, 8, ...] to be a tuple.
>

I would not use the word "tuple", in my experience, tuple in mathematics
(not computer science!) is only used in the way I described it: to gather
up the information about a structure into one object, so that we can say it
exists: because existing means some kind of set exists, and so we need to
somehow codify for e.g. addition on the integers both the addition and the
integers, i.e. combining two wholly different things into one 2-sequence:
(Z, +). Note that such structures might require infinite tuples, e.g. if
they support infinitely many operators. Anyway, this is where the
StackOverflow answer comes from: tuples are used in parlance for sequences
are in the same "space" for their coordinates, sequences for things that
have all coordinates in the same "space".

> In normal use, a tuple is considered to be an atomic[1] object (e.g. a
> point in space), while a sequence is, in a sense, a kind of iterative
> process that has been reified.
>
>
You can construct a sequence (or tuple) iteratively, but whether you do or
not has no bearing on the end result. Also, tuples are very much not atomic
in the mathematical sense. I would also like to note when you say "a tuple
is considered to be an atomic[1] object (e.g. a point in space)", then to a
mathematician, A_n = 1/sqrt(n) for n = 0, 1, ... is simply a point in space
too: just the space of sequences over the reals.

Mathematicians (generally, in the field of foundational logic it is a tad
different) don't tend to be concerned with differences such as how you
define an object (just need to make sure it exists), whether things are
finite or infinite, specified or unspecified. Unfortunately, in real life,
in a programming language, we do have to care about these things.

>
> > I'd say the difference is just one of semantics
>
> The difference between any two things is always one of semantics.
>
>
> > and as a mathematician I
> > would consider tuples and sequences as "isomorphic", in fact, the
> > set-theoretical construction of tuples as functions is *identical* to the
> > usual

[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Greg Ewing


On 6/05/20 6:12 pm, Steven D'Aprano wrote:


https://mathworld.wolfram.com/n-Tuple.html
https://mathworld.wolfram.com/List.html
https://mathworld.wolfram.com/Sequence.html


I get the feeling those definitions are a result of seeing
things through Mathematica-tinted glasses.

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Greg Ewing

On 6/05/20 1:58 pm, Henk-Jaap Wagenaar wrote:
I'd say the difference is just one of semantics and as a mathematician I
would consider tuples and sequences as "isomorphic", in fact, the
set-theoretical construction of tuples as functions is *identical* to
the usual definition of sequences: i.e. they are just two
interpretations of the the same object depending on your point of view.

Maybe the small subset of mathematicians that concern themselves
with trying to define everything in terms of sets, but I don't
think the majority of mathematicians think like that in their
everyday work. It's certainly at odds with the way I see tuples
and sequences being used in mathematics.

As well as the same type vs. different types thing, here are
some other salient differences:

- Infinite sequences make sense, infinite tuples not so much.

- Sequences are fundamentally ordered, whereas tuples are not
ordered in the same sense. Any apparent ordering in a tuple is
an artifact of the way we conventionally write them. If we were
in the habit of labelling the elements of a tuple and writing
things like (x:1, y:2, z:3) then we wouldn't have to write them
in any particular order -- (y:2, x:1, z:3) would be the same
tuple.

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-06 Thread Steven D'Aprano

On Wed, May 06, 2020 at 02:58:01AM +0100, Henk-Jaap Wagenaar wrote:

> I don't think that is accurate to represent as a representation of "a
> mathematician". The top voted answer here disagrees:
> https://math.stackexchange.com/questions/122595/whats-the-difference-between-tuples-and-sequences
> 
> "A sequence requires each element to be of the same type.
> A tuple can have elements with different types."

Are you saying that you can't have a sequence that alternates between 
ints and rationals, say, or ints and surds (reals)?

The sequence A_n = sqrt(n) from n=0 starts off int, int, real, ... so 
there is that.

For what its worth, Wolfram Mathworld disagrees with both Greg's comment 
and the stackexchange answer, stating that a tuple is just a synonym for 
a list, and that both lists and sequences are ordered sets:

https://mathworld.wolfram.com/n-Tuple.html

https://mathworld.wolfram.com/List.html

https://mathworld.wolfram.com/Sequence.html

> The common usage for both is: you have a tuple of (Z, +) representing the
> Abelian group of addition (+) on the integers (Z), whereas you have the
> sequence {1/n}_{n \in N} converging to 0 in the space Q^N (rational
> infinite sequences) for example.

One can come up with many other usages. I think a far more common use 
for tuples are the ordered pairs used for coordinates:

(1, 2)

So although tuples are ordered sets, and sequences are ordered sets, the 
way they are used is very different. One would not call the coordinate 
(1, 2) a sequence 1 followed by 2, and one would not normally consider a 
sequence such as [0, 2, 4, 6, 8, ...] to be a tuple.

In normal use, a tuple is considered to be an atomic[1] object (e.g. a 
point in space), while a sequence is, in a sense, a kind of iterative 
process that has been reified.

> I'd say the difference is just one of semantics 

The difference between any two things is always one of semantics.

> and as a mathematician I
> would consider tuples and sequences as "isomorphic", in fact, the
> set-theoretical construction of tuples as functions is *identical* to the
> usual definition of sequences: i.e. they are just two interpretations of
> the the same object depending on your point of view.

Many things are isomorphic. "Prime numbers greater than a googolplex" 
are isomorphic to the partial sums of the sequence 

1/2 − 1/4 + 1/8 − 1/16 + ⋯  = 1/3

but that doesn't mean you could use 1/2 * 1/4 as your RSA public key :-)

[1] I used that term intentionally, since we know that if you hit an 
atom hard enough, it ceases to be indivisible and can split apart :-)

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Steven D'Aprano

On Wed, May 06, 2020 at 12:39:30PM +1200, Greg Ewing wrote:
> On 6/05/20 2:22 am, jdve...@gmail.com wrote:
> >However, if sets and frozensets are "are considered to be
> >fundamentally the same kind of thing differentiated by mutability",
> >as you said, why not tuples and lists?
> 
> I think that can be answered by looking at the mathematical
> heritage of the types involved:
[...]
> To a mathematician, however, tuples and sequences are very
> different things. Python treating tuples as sequences is a
> "practicality beats purity" kind of thing, not to be expected
> from a mathematical point of view.

Thanks Greg, that's a really insightful observation.


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Henk-Jaap Wagenaar

On Wed, 6 May 2020 at 01:41, Greg Ewing  wrote:

> On 6/05/20 2:22 am, jdve...@gmail.com wrote:
> > However, if sets and frozensets are "are considered to be
> > fundamentally the same kind of thing differentiated by mutability",
> > as you said, why not tuples and lists?
>
> I think that can be answered by looking at the mathematical
> heritage of the types involved:
>
> Python Mathematics
> -- ---
> setset
> frozenset  set
> tuple  tuple
> list   sequence

>

> To a mathematician, however, tuples and sequences are very
> different things. Python treating tuples as sequences is a
> "practicality beats purity" kind of thing, not to be expected
> from a mathematical point of view.
>
>
I don't think that is accurate to represent as a representation of "a
mathematician". The top voted answer here disagrees:
https://math.stackexchange.com/questions/122595/whats-the-difference-between-tuples-and-sequences

"A sequence requires each element to be of the same type.
A tuple can have elements with different types."

The common usage for both is: you have a tuple of (Z, +) representing the
Abelian group of addition (+) on the integers (Z), whereas you have the
sequence {1/n}_{n \in N} converging to 0 in the space Q^N (rational
infinite sequences) for example.

I'd say the difference is just one of semantics and as a mathematician I
would consider tuples and sequences as "isomorphic", in fact, the
set-theoretical construction of tuples as functions is *identical* to the
usual definition of sequences: i.e. they are just two interpretations of
the the same object depending on your point of view.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Greg Ewing


On 6/05/20 2:22 am, jdve...@gmail.com wrote:

However, if sets and frozensets are "are considered to be
fundamentally the same kind of thing differentiated by mutability",
as you said, why not tuples and lists?


I think that can be answered by looking at the mathematical
heritage of the types involved:

Python Mathematics
-- ---
setset
frozenset  set
tuple  tuple
list   sequence

Sets and frozensets are both modelled after mathematical sets,
so to me at least it's not surprising that they behave very
similarly, and are interchangeable for many purposes.

To a mathematician, however, tuples and sequences are very
different things. Python treating tuples as sequences is a
"practicality beats purity" kind of thing, not to be expected
from a mathematical point of view.

--
Greg
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Ethan Furman


On 05/05/2020 03:05 AM, Alex Hall wrote:

On Tue, May 5, 2020 at 7:36 AM Raymond Hettinger mailto:raymond.hettin...@gmail.com>> wrote:

 >> Isn't a tuple essentially just a frozenlist? I know the intended
 >> semantics of tuples and lists tend to be different, but I'm not sure 
that's
 >> relevant.




Alex, please be careful of your quoting -- Raymond did not say the above 
paragraph.

--
~Ethan~
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread jdveiga

Steven D'Aprano wrote:
> On Tue, May 05, 2020 at 09:34:28AM -, jdve...@gmail.com wrote:
> > (frozenset() == set()) is True shocked
> > me.
> > According to wikipedia https://en.wikipedia.org/wiki/Equality_(mathematics):
> > "equality is a relationship between two quantities or, more generally two 
> > mathematical
> > expressions, asserting that the quantities have the same value, or that the 
> > expressions
> > represent the same mathematical object."
> > If lists and tuples are considered different "mathematical objects" 
> > (different types), they cannot be considered equal --tough they can be 
> > equivalent, for instance ([1, 2, 3] == list((1, 2, 3)) and tuple([1, 
> > 2, 3]) == (1, 2, 3)) is True.
> > There is no good correspondence between "mathematical objects" and 
> types. Even in mathematics, it is not clear whether the integer 1 as the 
> same mathematical object as the real number 1, or the complex number 1, 
> or the quaternion 1.
> In Python, we usually say that if a type is part of the numeric tower 
> ABC, then instances with the same numeric value should be considered 
> equal even if they have different types. But that's not a hard rule, 
> just a guideline.
> And it certainly shouldn't be used as a precedent implying that 
> non-numeric values should behave the same way.
> If you are looking for a single overriding consistant principle for 
> equality in Python, I think you are going to be disappointed. Python 
> does not pretend to be a formal mathematically consistent language 
> and the only principle for equality in Python is that equality means 
> whatever the object's __eq__ method wants it to mean.
> > I can only explain (frozenset() == set()) is
> > True vs (list() == tuple()) is False if:
> > a) frozensets and sets are considered the same "mathematical
> > 
> > objects". So immutability vs mutability is not a relevant feature in 
> > Python equality context. Then, list() == tuple() should be True
> > if 
> > no other feature distinguishes lists from tuples, I suppose...
> > List and tuple are distinguished by the most important feature of all: 
> the designer's intent. Tuples are records or structs, not frozen lists, 
> which is why they are called tuple not frozen list :-) even if people 
> use them as a defacto frozen list.
> On the other hand, frozensets are frozen sets, which is why they compare 
> equal.
> Does this make 100% perfectly logical sense? Probably not. But it 
> doesn't have to. Lists and tuples are considered to be independent kinds 
> of thing, while sets and frozensets are considered to be fundamentally 
> the same kind of thing differentiated by mutability.
> (In hindsight, it might have been more logically clear if mutable sets 
> inherited from immutable frozensets, but we missed the chance to do 
> that.)

Thanks for your reply.

I do not expect any kind of full correspondence between mathematical objects 
and programming objects. Just reasoning by analogy and trying to understand how 
lists and tuples cannot be equal and frozensets and sets can be on similar 
grounds. Mostly asking than answering.

Designers' intent is an admissible answer, of course. A cat and a dog can be 
equal if equality is defined as "having the same name".

However, designers' intent is one thing, and users' understating is another one.

>From your words, I have learnt that --from designers' point of view-- tuples 
>are different from lists in their nature while sets and frozensets are mostly 
>the same kind of thing --roughly speaking of course...

I wonder if users share that view. I feel that it is not unreasonable to expect 
that frozenset and set cannot be equal on the grounds that they are different 
types (as tuples and lists are different types too). From that perspective, 
equality on tuples / lists and frozensets / sets should follow similar rules. 
Not being that way is surprising. That is all.

However, if sets and frozensets are "are considered to be fundamentally the 
same kind of thing differentiated by mutability", as you said, why not tuples 
and lists? And that is, I guess, the reasoning behind proponent's claim. What 
if the difference between tuples and lists is not so deep or relevant and they 
just differ on mutability?

Asking again...
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Steven D'Aprano

On Tue, May 05, 2020 at 09:34:28AM -, jdve...@gmail.com wrote:

> `(frozenset() == set()) is True` shocked me.
> 
> According to wikipedia https://en.wikipedia.org/wiki/Equality_(mathematics): 
> "equality is a relationship between two quantities or, more generally two 
> mathematical expressions, asserting that the quantities have the same value, 
> or that the expressions represent the same mathematical object."
> 

> If lists and tuples are considered different "mathematical objects" 
> (different types), they cannot be considered equal --tough they can be 
> equivalent, for instance `([1, 2, 3] == list((1, 2, 3)) and tuple([1, 
> 2, 3]) == (1, 2, 3)) is True`.

There is no good correspondence between "mathematical objects" and 
types. Even in mathematics, it is not clear whether the integer 1 as the 
same mathematical object as the real number 1, or the complex number 1, 
or the quaternion 1.

In Python, we usually say that if a type is part of the numeric tower 
ABC, then instances with the same numeric value should be considered 
equal even if they have different types. But that's not a hard rule, 
just a guideline.

And it certainly shouldn't be used as a precedent implying that 
non-numeric values should behave the same way.

If you are looking for a single overriding consistant principle for 
equality in Python, I think you are going to be disappointed. Python 
does not pretend to be a formal mathematically consistent language 
and the only principle for equality in Python is that equality means 
whatever the object's `__eq__` method wants it to mean.

> I can only explain `(frozenset() == set()) is True` vs `(list() == tuple()) 
> is False` if:
> 
> a) `frozenset`s and `set`s are considered the same "mathematical 
> objects". So immutability vs mutability is not a relevant feature in 
> Python equality context. Then, `list() == tuple()` should be `True` if 
> no other feature distinguishes lists from tuples, I suppose...

List and tuple are distinguished by the most important feature of all: 
the designer's intent. Tuples are records or structs, not frozen lists, 
which is why they are called tuple not frozen list :-) even if people 
use them as a defacto frozen list.

On the other hand, frozensets are frozen sets, which is why they compare 
equal.

Does this make 100% perfectly logical sense? Probably not. But it 
doesn't have to. Lists and tuples are considered to be independent kinds 
of thing, while sets and frozensets are considered to be fundamentally 
the same kind of thing differentiated by mutability.

(In hindsight, it might have been more logically clear if mutable sets 
inherited from immutable frozensets, but we missed the chance to do 
that.)

-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread Alex Hall

On Tue, May 5, 2020 at 7:36 AM Raymond Hettinger <
raymond.hettin...@gmail.com> wrote:

> >> Isn't a tuple essentially just a frozenlist? I know the intended
> >> semantics of tuples and lists tend to be different, but I'm not sure
> that's
> >> relevant.
>
>
> In terms of API, it might look that way.  But in terms of use cases, they
> are less alike:  lists-are-looping, tuples-are-for-nonhomongenous-fields.
> List are like database tables; tuples are like records in the database.
>  Lists are like C arrays; tuples are like structs.
>

Right, that's what I'm referring to. If you're comparing two things which
are meant to represent completely different entities (say, comparing a
record to a table) then your code is probably completely broken (why would
you be doing that?) and having equality return False isn't going to fix
that. Conversely I can't see how returning True could break a program that
would work correctly otherwise.

If you're comparing a list and a tuple, and you haven't completely screwed
up, you probably mean to compare the elements and you made a small mistake,
e.g. you used the wrong brackets, or you forgot that *args produces a tuple.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-05 Thread jdveiga

Raymond Hettinger wrote:
> > On May 3, 2020, at 6:19 PM, Steven D'Aprano st...@pearwood.info wrote:
> > frozenset and set make a counterexample:
> > frozenset({1}) == {1}
> > True
> > Nice catch! That's really interesting. Is there reasoning
> > behind
> > frozenset({1}) == {1} but [1] != (1,), or is it just an accident
> > of
> > history?
> > Conceptually, sets are sets, whether they are mutable or frozen.
> > Right.  This isn't an accident. It is by design.
> Also, some numeric types are specifically designed for cross-type comparison:
>  >>> int(3) == float(3) == complex(3, 0)
>  True
> 
> And in Python 2, by design, str and unicode were comparable:
> >>> u'abc' == 'abc'
> True
> 
> But the general rule is that objects aren't cross-type comparable by default. 
>  We have
> to specifically enable that behavior when we think it universally makes 
> sense.  The modern
> trend is to avoid cross-type comparability, enumerates and data classes for 
> example:
> >>> Furniture = Enum('Furniture', ('table', 'chair', 'couch'))
> >>> HTML = Enum('HTML', ('dl', 'ol', 'ul', 'table'))
> >>> Furniture.table == HTML.table
> False
> 
> >>> A = make_dataclass('A', 'x')
> >>> B = make_dataclass('B', 'x')
> >>> A(10) == B(10)
> False
> 
> Bytes and str are not comparable in Python 3:
> >>> b'abc' == 'abc'
> False
> 
> > Isn't a tuple
> > essentially just a frozenlist? I know the intended
> > semantics of tuples and lists tend to be different, but I'm not sure that's
> > relevant.
> > In terms of API, it might look that way.  But in terms of use cases, they
> are less alike:  lists-are-looping, tuples-are-for-nonhomongenous-fields.  
> List are like
> database tables; tuples are like records in the database.   Lists are like C 
> arrays;
> tuples are like structs.
> On the balance, I think more harm than good would result from making sequence 
> equality
> not depend on type.  Also when needed, it isn't difficult to be explicit that 
> you're
> converting to a common type to focus on contents:
> >>> s = bytes([10, 20, 30])
> >>> t = (10, 20, 30)
> >>> list(s) == list(t)
> 
> When you think about it, it makes sense that a user gets to choose whether 
> equality is
> determined by contents or by contents and type.  For some drinkers, a can of 
> beer is equal
> to a bottle of bear; for some drinkers, they aren't equal at all ;-)
> Lastly, when it comes to containers.  They each get to make their own rules 
> about what
> is equal.  Dicts compare on contents regardless of order, but OrderedDict 
> requires that
> the order matches.
> Raymond

`(frozenset() == set()) is True` shocked me.

According to wikipedia https://en.wikipedia.org/wiki/Equality_(mathematics): 
"equality is a relationship between two quantities or, more generally two 
mathematical expressions, asserting that the quantities have the same value, or 
that the expressions represent the same mathematical object."

If lists and tuples are considered different "mathematical objects" (different 
types), they cannot be considered equal --tough they can be equivalent, for 
instance `([1, 2, 3] == list((1, 2, 3)) and tuple([1, 2, 3]) == (1, 2, 3)) is 
True`.

I can only explain `(frozenset() == set()) is True` vs `(list() == tuple()) is 
False` if:

a) `frozenset`s and `set`s are considered the same "mathematical objects". So 
immutability vs mutability is not a relevant feature in Python equality 
context. Then, `list() == tuple()` should be `True` if no other feature 
distinguishes lists from tuples, I suppose...

b) language designers found `(frozenset() == set()) is True` convenient (why?). 
Then, why is not `(list() == tuple()) is True` so convenient?

c) it is a bug and `frozenset() == set()` should be `True`.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-04 Thread Raymond Hettinger

> On May 3, 2020, at 6:19 PM, Steven D'Aprano  wrote:
> 
>>> `frozenset` and `set` make a counterexample:
>>> 
>> frozenset({1}) == {1}
>>> True
>>> 
>> 
>> Nice catch! That's really interesting. Is there reasoning behind
>> `frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
>> history? 
> 
> Conceptually, sets are sets, whether they are mutable or frozen.

Right.  This isn't an accident. It is by design.

Also, some numeric types are specifically designed for cross-type comparison:

 >>> int(3) == float(3) == complex(3, 0)
 True

And in Python 2, by design, str and unicode were comparable:

>>> u'abc' == 'abc'
True

But the general rule is that objects aren't cross-type comparable by default.  
We have to specifically enable that behavior when we think it universally makes 
sense.  The modern trend is to avoid cross-type comparability, enumerates and 
data classes for example:

>>> Furniture = Enum('Furniture', ('table', 'chair', 'couch'))
>>> HTML = Enum('HTML', ('dl', 'ol', 'ul', 'table'))
>>> Furniture.table == HTML.table
False

>>> A = make_dataclass('A', 'x')
>>> B = make_dataclass('B', 'x')
>>> A(10) == B(10)
False

Bytes and str are not comparable in Python 3:

>>> b'abc' == 'abc'
False

>> Isn't a tuple essentially just a frozenlist? I know the intended
>> semantics of tuples and lists tend to be different, but I'm not sure that's
>> relevant.

In terms of API, it might look that way.  But in terms of use cases, they are 
less alike:  lists-are-looping, tuples-are-for-nonhomongenous-fields.  List are 
like database tables; tuples are like records in the database.   Lists are like 
C arrays; tuples are like structs.

On the balance, I think more harm than good would result from making sequence 
equality not depend on type.  Also when needed, it isn't difficult to be 
explicit that you're converting to a common type to focus on contents:

>>> s = bytes([10, 20, 30])
>>> t = (10, 20, 30)
>>> list(s) == list(t)

When you think about it, it makes sense that a user gets to choose whether 
equality is determined by contents or by contents and type.  For some drinkers, 
a can of beer is equal to a bottle of bear; for some drinkers, they aren't 
equal at all ;-)

Lastly, when it comes to containers.  They each get to make their own rules 
about what is equal.  Dicts compare on contents regardless of order, but 
OrderedDict requires that the order matches.

Raymond

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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-03 Thread Chris Angelico

On Mon, May 4, 2020 at 11:43 AM Soni L.  wrote:
>
>
>
> On 2020-05-03 10:19 p.m., Steven D'Aprano wrote:
> > On Sat, May 02, 2020 at 11:01:21PM +0200, Alex Hall wrote:
> > > On Sat, May 2, 2020 at 10:52 PM Dominik Vilsmeier 
> > > 
> > > wrote:
> > >
> > > > `frozenset` and `set` make a counterexample:
> > > >
> > > > >>> frozenset({1}) == {1}
> > > > True
> > > >
> > >
> > > Nice catch! That's really interesting. Is there reasoning behind
> > > `frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
> > > history?
> >
> > Conceptually, sets are sets, whether they are mutable or frozen.
> >
> >
> > > Isn't a tuple essentially just a frozenlist? I know the intended
> > > semantics of tuples and lists tend to be different, but I'm not sure 
> > > that's
> > > relevant.
> >
> > o_O
> >
> > If the intended semantics aren't relevant, I'm not sure what is...
> >
> >
>
> for what it's worth, I see myself using tuples as frozen lists more
> often than their "intended semantics".
>
> more specifically, you can't pass lists to:
>
> 1. isinstance
> 2. issubclass
> 3. str.endswith
>
> among others. so I sometimes just convert a list of strings into a tuple
> of strings and store it somewhere so I can use it with str.endswith
> later. (this is not how you're "supposed" to implement domain suffix
> blocks but w/e)

That doesn't mean you're using a tuple as a frozen list - it means
you're using a tuple as a static collection. I've never had a
situation where I've wanted to use isinstance with a list that gets
built progressively at run-time; it's always a prewritten collection.

I don't see what this has to do with lists and tuples. You're using
tuples the way they're meant to be used.

ChrisA
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-03 Thread Soni L.

On 2020-05-03 10:19 p.m., Steven D'Aprano wrote:

On Sat, May 02, 2020 at 11:01:21PM +0200, Alex Hall wrote:
> On Sat, May 2, 2020 at 10:52 PM Dominik Vilsmeier 
> wrote:
> 
> > `frozenset` and `set` make a counterexample:

> >
> > >>> frozenset({1}) == {1}
> > True
> >
> 
> Nice catch! That's really interesting. Is there reasoning behind

> `frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
> history? 

Conceptually, sets are sets, whether they are mutable or frozen.

> Isn't a tuple essentially just a frozenlist? I know the intended
> semantics of tuples and lists tend to be different, but I'm not sure that's
> relevant.

o_O

If the intended semantics aren't relevant, I'm not sure what is...

for what it's worth, I see myself using tuples as frozen lists more 
often than their "intended semantics".

more specifically, you can't pass lists to:

1. isinstance
2. issubclass
3. str.endswith

among others. so I sometimes just convert a list of strings into a tuple 
of strings and store it somewhere so I can use it with str.endswith 
later. (this is not how you're "supposed" to implement domain suffix 
blocks but w/e)

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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-03 Thread Steven D'Aprano

On Sat, May 02, 2020 at 11:01:21PM +0200, Alex Hall wrote:
> On Sat, May 2, 2020 at 10:52 PM Dominik Vilsmeier 
> wrote:
> 
> > `frozenset` and `set` make a counterexample:
> >
> > >>> frozenset({1}) == {1}
> > True
> >
> 
> Nice catch! That's really interesting. Is there reasoning behind
> `frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
> history? 

Conceptually, sets are sets, whether they are mutable or frozen.


> Isn't a tuple essentially just a frozenlist? I know the intended
> semantics of tuples and lists tend to be different, but I'm not sure that's
> relevant.

o_O

If the intended semantics aren't relevant, I'm not sure what is...


-- 
Steven
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-03 Thread Richard Damon

On 5/3/20 8:40 AM, Ahmed Amr wrote:
> Thanks, I do appreciate all the discussion here about that.
>
> Initially, I was thinking about having lists/arrays/tuples match the behavior 
> of other instances in python that compare across their types like:
> 1) Sets (instances of set or frozenset) can be compared within and across 
> their types As Dominic mentioned.
> 2) Numeric types do compare across their types along with fractions.Fraction 
> and decimal.Decimal.
> 3) Binary Sequences( instances of bytes or bytearray) can be compared within 
> and across their types
> (All points above stated in python reference in 
> https://docs.python.org/3/reference/expressions.html)
>
> but after the discussion here, I think backword compatibility dominates for 
> sure against that, Thanks!

I think the issue is that the set/frozen set distinction (and
bytes/bytes array) is a much finer distinction than between arbitrary
sequence types, as it is primarily just a change of mutability (and
hash-ability), and all the Numeric types are really just slight
different abstractions of the same basic set of values (or subsets thereof).

The various containers don't have the same concept that they are
essentially representing the same 'thing' with just a change in
representation to control the types sort of numbers they can express and
what sort of numeric errors the might contain (so two representations
that map to the same abstract number make sense to be equal)

Different types of sequences are more different in what they likely
represent, so it is less natural for different sequences of the same
value to be thought of as always being 'the same'

There may be enough cases where that equality is reasonable, that having
a 'standard' function to perform that comparison might make sense, it
just isn't likely to be spelled ==.

There are several questions on how to do thing that might need to be
explored, Should the ignoring of sequence type be recurcively ignored or
not, i.e. is [1, [2, 3]] the same as (1, (2, 3)) or not, and are strings
just another sequence type, or something more fundamental. This doesn't
make it a 'bad' idea, just a bit more complicated and in need of
exploration.

-- 
Richard Damon
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-03 Thread Ahmed Amr

Thanks, I do appreciate all the discussion here about that.

Initially, I was thinking about having lists/arrays/tuples match the behavior 
of other instances in python that compare across their types like:
1) Sets (instances of set or frozenset) can be compared within and across their 
types As Dominic mentioned.
2) Numeric types do compare across their types along with fractions.Fraction 
and decimal.Decimal.
3) Binary Sequences( instances of bytes or bytearray) can be compared within 
and across their types
(All points above stated in python reference in 
https://docs.python.org/3/reference/expressions.html)

but after the discussion here, I think backword compatibility dominates for 
sure against that, Thanks!
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Eric V. Smith


On 5/2/2020 6:03 PM, Alex Hall wrote:
On Sat, May 2, 2020 at 11:39 PM Richard Damon 
mailto:rich...@damon-family.org>> wrote:


Doesn't it also imply that ('a',) == 'a' and that you couldn't get a
dict to use both of these as keys?


```
>>> {1.0: 2, 1: 3}
{1.0: 3}
```


I think the implication is that if ('a',) == 'a' it would cause a 
breaking change, whereas the float / integer behavior is existing.


Eric

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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, May 2, 2020 at 11:39 PM Richard Damon 
wrote:

> Doesn't it also imply that ('a',) == 'a' and that you couldn't get a
> dict to use both of these as keys?
>

```
>>> {1.0: 2, 1: 3}
{1.0: 3}
```
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Richard Damon

On 5/2/20 5:13 PM, Serhiy Storchaka wrote:
> 02.05.20 23:32, Alex Hall пише:
>> On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka > > wrote:
>>     02.05.20 21:34, Ahmed Amr пише:
>>  > I see there are ways to compare them item-wise, I'm suggesting to
>>     bake
>>  > that functionality inside the core implementation of such indexed
>>  > structures.
>>  > Also those solutions are direct with tuples and lists, but it
>>     wouldn't
>>  > be as direct with arrays-lists/tuples comparisons for example.
>>
>>     If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1,
>> 2, 3))
>>     == hash([1, 2, 3])`.
>>
>>
>> Would we? Is the contract `x == y => hash(x) == hash(y)` still
>> required if hash(y) is an error? What situation involving dicts could
>> lead to a bug if `(1, 2, 3) == [1, 2, 3]` but `hash((1, 2, 3))` is
>> defined and `hash([1, 2, 3])` isn't?
>>
>> The closest example I can think of is that you might think you can do
>> `{(1, 2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError: unhashable
>> type: 'list'` it'd be easy to fix.
>
> You are probably right.
>
> Here is other example: if make all sequences comparable by content, we
> would need to make `('a', 'b', 'c') == 'abc'` and `hash(('a', 'b',
> 'c')) == hash('abc')`. It may be deifficult to get the latter taking
> into account hash randomization. 
Doesn't it also imply that ('a',) == 'a' and that you couldn't get a
dict to use both of these as keys?

-- 
Richard Damon
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Serhiy Storchaka


02.05.20 23:32, Alex Hall пише:
On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka 
> wrote:

02.05.20 21:34, Ahmed Amr пише:
 > I see there are ways to compare them item-wise, I'm suggesting to
bake
 > that functionality inside the core implementation of such indexed
 > structures.
 > Also those solutions are direct with tuples and lists, but it
wouldn't
 > be as direct with arrays-lists/tuples comparisons for example.

If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
== hash([1, 2, 3])`.


Would we? Is the contract `x == y => hash(x) == hash(y)` still required 
if hash(y) is an error? What situation involving dicts could lead to a 
bug if `(1, 2, 3) == [1, 2, 3]` but `hash((1, 2, 3))` is defined and 
`hash([1, 2, 3])` isn't?


The closest example I can think of is that you might think you can do 
`{(1, 2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError: unhashable 
type: 'list'` it'd be easy to fix.


You are probably right.

Here is other example: if make all sequences comparable by content, we 
would need to make `('a', 'b', 'c') == 'abc'` and `hash(('a', 'b', 'c')) 
== hash('abc')`. It may be deifficult to get the latter taking into 
account hash randomization.

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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, May 2, 2020 at 10:52 PM Dominik Vilsmeier 
wrote:

> `frozenset` and `set` make a counterexample:
>
> >>> frozenset({1}) == {1}
> True
>

Nice catch! That's really interesting. Is there reasoning behind
`frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
history? Isn't a tuple essentially just a frozenlist? I know the intended
semantics of tuples and lists tend to be different, but I'm not sure that's
relevant.
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, May 2, 2020 at 10:36 PM Guido van Rossum  wrote:

> It does look like that would violate a basic property of `==` -- if two
> values compare equal, they should be equally usable as dict keys.
>

It's certainly a reasonable property, but I don't think it's critical.

By comparison, if it was the case that `(1, 2, 3) == [1, 2, 3]` and
`hash((1, 2, 3)) != hash([1, 2, 3])` were both True without raising
exceptions, that would be a disaster and lead to awful bugs. The
equality/hash contract is meant to protect against that.

> I can't think of any counterexamples.
>

I think it's reasonable that this change would introduce counterexamples
where none previously existed, as we would be changing the meaning of ==.
Although since writing this Dominik gave the frozenset example.

I also think it'd be possible to have a data model where `{(1, 2, 3):
4}[[1, 2, 3]]` does work. You'd need a way to calculate a hash if you
promised to use it only for `__getitem__`, not `__setitem__`, so you can't
store list keys but you can access with them.

(this is all just fun theoretical discussion, I'm still not supporting the
proposal)

>
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Guido van Rossum

Okay, that's fair. So the argument really comes down to backwards
compatibility (which is inconvenient but important).

On Sat, May 2, 2020 at 1:51 PM Dominik Vilsmeier 
wrote:

> `frozenset` and `set` make a counterexample:
>
> >>> frozenset({1}) == {1}
> True
> On 02.05.20 22:36, Guido van Rossum wrote:
>
> It does look like that would violate a basic property of `==` -- if two
> values compare equal, they should be equally usable as dict keys. I can't
> think of any counterexamples.
>
> On Sat, May 2, 2020 at 1:33 PM Alex Hall  wrote:
>
>> On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka 
>> wrote:
>>
>>> 02.05.20 21:34, Ahmed Amr пише:
>>> > I see there are ways to compare them item-wise, I'm suggesting to bake
>>> > that functionality inside the core implementation of such indexed
>>> > structures.
>>> > Also those solutions are direct with tuples and lists, but it wouldn't
>>> > be as direct with arrays-lists/tuples comparisons for example.
>>>
>>> If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
>>> == hash([1, 2, 3])`.
>>>
>>
>> Would we? Is the contract `x == y => hash(x) == hash(y)` still required
>> if hash(y) is an error? What situation involving dicts could lead to a bug
>> if `(1, 2, 3) == [1, 2, 3]` but `hash((1, 2, 3))` is defined and `hash([1,
>> 2, 3])` isn't?
>>
>> The closest example I can think of is that you might think you can do
>> `{(1, 2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError: unhashable type:
>> 'list'` it'd be easy to fix.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Dominik Vilsmeier


`frozenset` and `set` make a counterexample:

>>> frozenset({1}) == {1}
True

On 02.05.20 22:36, Guido van Rossum wrote:

It does look like that would violate a basic property of `==` -- if
two values compare equal, they should be equally usable as dict keys.
I can't think of any counterexamples.

On Sat, May 2, 2020 at 1:33 PM Alex Hall mailto:alex.moj...@gmail.com>> wrote:

On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka
mailto:storch...@gmail.com>> wrote:

02.05.20 21:34, Ahmed Amr пише:
> I see there are ways to compare them item-wise, I'm
suggesting to bake
> that functionality inside the core implementation of such
indexed
> structures.
> Also those solutions are direct with tuples and lists, but
it wouldn't
> be as direct with arrays-lists/tuples comparisons for example.

If make `(1, 2, 3) == [1, 2, 3]` we would need to make
`hash((1, 2, 3))
== hash([1, 2, 3])`.


Would we? Is the contract `x == y => hash(x) == hash(y)` still
required if hash(y) is an error? What situation involving dicts
could lead to a bug if `(1, 2, 3) == [1, 2, 3]` but `hash((1, 2,
3))` is defined and `hash([1, 2, 3])` isn't?

The closest example I can think of is that you might think you can
do `{(1, 2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError:
unhashable type: 'list'` it'd be easy to fix.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Guido van Rossum

It does look like that would violate a basic property of `==` -- if two
values compare equal, they should be equally usable as dict keys. I can't
think of any counterexamples.

On Sat, May 2, 2020 at 1:33 PM Alex Hall  wrote:

> On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka 
> wrote:
>
>> 02.05.20 21:34, Ahmed Amr пише:
>> > I see there are ways to compare them item-wise, I'm suggesting to bake
>> > that functionality inside the core implementation of such indexed
>> > structures.
>> > Also those solutions are direct with tuples and lists, but it wouldn't
>> > be as direct with arrays-lists/tuples comparisons for example.
>>
>> If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
>> == hash([1, 2, 3])`.
>>
>
> Would we? Is the contract `x == y => hash(x) == hash(y)` still required if
> hash(y) is an error? What situation involving dicts could lead to a bug if
> `(1, 2, 3) == [1, 2, 3]` but `hash((1, 2, 3))` is defined and `hash([1, 2,
> 3])` isn't?
>
> The closest example I can think of is that you might think you can do
> `{(1, 2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError: unhashable type:
> 'list'` it'd be easy to fix.
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[Python-ideas] Re: Equality between some of the indexed collections

On Sat, May 2, 2020 at 9:51 PM Serhiy Storchaka  wrote:

> 02.05.20 21:34, Ahmed Amr пише:
> > I see there are ways to compare them item-wise, I'm suggesting to bake
> > that functionality inside the core implementation of such indexed
> > structures.
> > Also those solutions are direct with tuples and lists, but it wouldn't
> > be as direct with arrays-lists/tuples comparisons for example.
>
> If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3))
> == hash([1, 2, 3])`.
>

Would we? Is the contract `x == y => hash(x) == hash(y)` still required if
hash(y) is an error? What situation involving dicts could lead to a bug if
`(1, 2, 3) == [1, 2, 3]` but `hash((1, 2, 3))` is defined and `hash([1, 2,
3])` isn't?

The closest example I can think of is that you might think you can do `{(1,
2, 3): 4}[[1, 2, 3]]`, but once you get `TypeError: unhashable type:
'list'` it'd be easy to fix.
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[Python-ideas] Re: Equality between some of the indexed collections

2020-05-02 Thread Serhiy Storchaka


02.05.20 21:34, Ahmed Amr пише:
I see there are ways to compare them item-wise, I'm suggesting to bake 
that functionality inside the core implementation of such indexed 
structures.
Also those solutions are direct with tuples and lists, but it wouldn't 
be as direct with arrays-lists/tuples comparisons for example.


If make `(1, 2, 3) == [1, 2, 3]` we would need to make `hash((1, 2, 3)) 
== hash([1, 2, 3])`.

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[Python-ideas] Re: Equality between some of the indexed collections