[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

Terry J. Reedy  added the comment:

Martin: [tests need review].  Yep.  Improved constant folding and elimination 
of duplication now causes many 'is not' tests to fail.  Language tests should 
not test current implementation limitations.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Humbdrag]

Change by Humbdrag :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Humbled Drugman]

Change by Humbled Drugman :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Humbled Drugman]

Change by Humbled Drugman :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Roundup Robot]

Change by Roundup Robot :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Cheryl Sabella]

Cheryl Sabella added the comment:

I've created a PR for the changes to test_compare from v16 of the patch.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Cheryl Sabella]

Changes by Cheryl Sabella :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Guido van Rossum]

Changes by Guido van Rossum :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Yes I think I committed all the documentation. Someone needs to decide whether 
to use Andy’s tests as they are, or perhaps modify or drop some or all of them.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Cheryl Sabella]

Cheryl Sabella added the comment:

It appears all the patches for this issue have been applied.  Is the only open 
item the changes to test_compare?

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8c9a86aa222e by Martin Panter in branch '3.5':
Issue #12067: Recommend that hash and equality be consistent
https://hg.python.org/cpython/rev/8c9a86aa222e

New changeset 9702c5f08df1 by Martin Panter in branch '3.6':
Issues #12067: Merge hash recommendation from 3.5
https://hg.python.org/cpython/rev/9702c5f08df1

New changeset 9dbb7bbc1449 by Martin Panter in branch 'default':
Issues #12067: Merge hash recommendation from 3.6
https://hg.python.org/cpython/rev/9dbb7bbc1449

New changeset 8a9904c5cb1d by Martin Panter in branch '2.7':
Issue #12067: Rewrite Comparisons section in the language reference
https://hg.python.org/cpython/rev/8a9904c5cb1d

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Changes by Martin Panter :


Added file: http://bugs.python.org/file46399/expressions-py3.7_v17.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Updated patch for 2.7, which I plan to commit soon. Corresponding Py 3 patch 
coming soon.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Here is a port of the documentation to Python 2. Main differences:

* Default rules for order comparisons are different
* Not all kinds of objects inherit from object()
* str(), unicode() compatibility
* xrange() only seems to have default comparability
* NAN, “binary sequences” and sets not listed

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Changes by Martin Panter :


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

I committed the changes to expressions.rst for 3.4+. That still leaves the 
changes to test_compare.py, and possibly changes for 2.7.

Andy: In msg229721 you mentioned a potential 2.7 patch. Did you get anywhere 
with that? Even if it is only half finished, someone else may be able to keep 
working on it.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Roundup Robot]

Roundup Robot added the comment:

New changeset 1fc049e5ec14 by Martin Panter in branch '3.4':
Issue #12067: Rewrite Comparisons section in the language reference
https://hg.python.org/cpython/rev/1fc049e5ec14

New changeset b6698c00265b by Martin Panter in branch '3.5':
Issue #12067: Merge comparisons doc from 3.4 into 3.5
https://hg.python.org/cpython/rev/b6698c00265b

New changeset 294b8a7957e9 by Martin Panter in branch 'default':
Issue #12067: Merge comparisons doc from 3.5
https://hg.python.org/cpython/rev/294b8a7957e9

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

I can split out a documentation-only patch if it would help get that committed.

In the meantime, patch v16 includes some fixups to comments etc in the test 
code that I missed myself.

--
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components: +Tests
Added file: 
http://bugs.python.org/file40052/issue12067-expressions-py3.6_v16.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Patch v15. No doc changes, but I refactored the test code:

* Manually merged with recent changes
* Separate assert_equality_only() and assert_total_order() test methods. 
Hopefully this is a bit simpler for people to understand and review, and avoids 
suggesting that partial ordering is tested.
* Dropped subTest() instances with identical parameters. The basic stack trace 
can already distinguish these.
* Eliminated is_value_comparable(); remove the ”meth” parameters from the call 
sites instead

--
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Added file: 
http://bugs.python.org/file39970/issue12067-expressions-py3.5_v15.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Berker Peksag]

Berker Peksag added the comment:

I think we can commit documentation and tests separately. I just did a quick 
review of the test changes and I will add some review comments later (sorry, 
lack of time :)).

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Ethan Furman]

Changes by Ethan Furman et...@stoneleaf.us:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

I have posted v14 of the patch (for the 3.5 'default' branch), based on 
Martin's v13. v14 addresses all comments Martin made, as described in my 
responses to them (see patch set 10).

On Issue 4395: That issue should be pursued in addition to this issue; it seems 
Martin's patch for it is complementary to the patch for this issue here.

On Issue 22000: I agree that that issue is addressed by the patch for this 
issue here.

All: Please review the v14 patch.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


Added file: 
http://bugs.python.org/file38303/issue12067-expressions-py3.5_v14.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Issue 4395 is already open for the != delegating to __eq__ issue that Guido 
pointed out earlier.

Yet another issue that this doc patch should solve: Issue 22000.

I am posting v13 of the patch that works with the current “default” (3.5) 
branch. Minor modifications to the documentation part:

* Undo some paragraph reflowing to reduce the size of the diff hunks
* Use math.nan instead of nan = float('NaN')
* Tweaked the new subheadings to better differentiate “Value comparisons” 
(which this bug is concerned with) from “Identity comparisons”

Changes I made to the added test cases:

* Drop Python 3 version check
* Use TestCase.subTest() so it’s easier to see everything that [is] was failing
* Merge some tests for != and fix changed expectations due to Issue 21408 being 
fixed
* Drop _inst_str() and __str__(), apparently unused

It would be nice to see at least the documentation part reviewed and committed, 
even if the tests require more work. (After having to hack the tests to get 
them working, I might point out some odd bits in the code review.) Though I 
guess they are just tests, so it doesn’t matter so much as long as they pass.

--
Added file: 
http://bugs.python.org/file38183/issue12067-expressions-py3.5_v13.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

I have posted v12 of the patch, which addresses all comments since v11.

This Python 3.4 patch can be applied to the default (3.5 dev) branch as well.

I will start working on a similar patch for Python 2.7 now.

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Added file: 
http://bugs.python.org/file36978/issue12067-expressions-py34_v12.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

I have addressed the comments by Jim Jewett, Martin Panter and of myself in a 
new version v11, which got posted.

For the expression.rst doc file, this version of the patch has its diff 
sections in a logical order, so that the original text and the patched text are 
close by each other.

Please review.

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Added file: 
http://bugs.python.org/file36920/issue12067-expressions-py34_v11.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

I also made sure in both files that the line length of any changed or new lines 
is max 80. Sorry if that creates extra changes when looking at deltas between 
change sets.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

@Guido:
Agree to all you said in your #msg226496.

There is additional information about comparison in:
- Tutorial (5.8. Comparing Sequences and Other Types),
- Library Reference (5.3. Comparisons),
- Language Reference (3.3.1. Basic customization)
that needs to be reviewed in light of this patch.

I'm just not sure I want to make this patch even larger as it is already, and 
tend to do that in a follow on issue and patch (unless directed otherwise).

Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploading v10 of the patch, which addresses all review comments made on v9.

There is one open question back to Martin Panter about which different types of 
byte sequences can be compared in Py 3.4.

I also believe this patch addresses all of Issue 22001. Let me know if you find 
that that is not the case.

If we continue to scope this patch to only the comparison chapter of the 
language reference, then I think we are done (see msg229229 about other places 
that need review and possibly updates).

Please review the patch v10.

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Added file: 
http://bugs.python.org/file36896/issue12067-expressions-py34_v10.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Here is the delta between v9 and v10 of the patch, if people want to see just 
that.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

About the byte sequence comparisons, I wondered if it was misleading to say 
that a list(), tuple() or range() can only be compared to the same type, 
without mentioning that bytes() and bytearray() can be compared to each other.

BTW just noticed you say range() supports lexicographical ordering, which I 
think is incorrect.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

Maybe it would be wise to split this task up and commit the bits that don’t 
need any more work. I think the existing patch might already solve Issue 22001.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Just wanted to say that i will continue working on this, working in the 
comments made so far...
Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Guido van Rossum]

Guido van Rossum added the comment:

- This bug should discuss doc updates, not question the rules.

- The rules have evolved over time and the docs stayed behind.

- We should definitely update the 2.7 docs as well as the 3.4 and 3.5 (in 
development) docs.  The 2.7 docs need to be different than the 3.x docs.

- The language reference manual should clearly state the rules so that 
implementers can use them as guidelines for implementation.

- There are several sets of relevant rules:

(a) How is each operator translated into a series of lookups and method calls, 
etc.  It's similar to other binary operators except that the reverse for __lt__ 
is __gt__ instead of __rlt__, and there's an extra rule that if __ne__ doesn't 
exist we compute __eq__ and take the opposite.

(b) The default implementation (e.g. default == falls back to 'is',  raises 
TypeError).

(c) The rules for built-in types, especially numbers (if there are still 
special cases that aren't explained by the __xx__ methods on the various 
numeric types).

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Martin Panter added the comment:

The point about “a != b” deferring to “not a.__eq__(b)” is not documented 
anywhere that I am aware of. In fact the opposite is currently documented at 
https://docs.python.org/release/3.4.0/reference/datamodel.html#richcmpfuncs, 
so maybe this needs to be fixed, one way or another.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Guido van Rossum]

Guido van Rossum added the comment:

That's a pretty new feature. Someone probably forgot to clean up all the
places where it was documented.

On Sat, Sep 6, 2014 at 4:18 PM, Martin Panter rep...@bugs.python.org
wrote:


 Martin Panter added the comment:

 The point about “a != b” deferring to “not a.__eq__(b)” is not documented
 anywhere that I am aware of. In fact the opposite is currently documented
 at 
 https://docs.python.org/release/3.4.0/reference/datamodel.html#richcmpfuncs,
 so maybe this needs to be fixed, one way or another.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Martin Panter]

Changes by Martin Panter vadmium...@gmail.com:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Berker Peksag]

Changes by Berker Peksag berker.pek...@gmail.com:


--
stage: needs patch - patch review

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploaded v9 of the patch for 3.4 and default.

It reflects Marc's comment, plus the result of the recent discussion on 
python-dev since v8 of th epatch, up to 2014-07-15 (subject: == on object tests 
identity in 3.x).

- Please review the patch.

--
Added file: http://bugs.python.org/file35971/issue12067-expressions-py34_v9.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


--
assignee: rhettinger - 

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


Added file: http://bugs.python.org/file35938/try_eq.py

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


Added file: http://bugs.python.org/file35939/try_eq.out

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Mark: Both are good points!

Would you add the cases from your second comment under symmetry?

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploaded v8 of the patch for 3.4 and default.

It reflects hopefully everything that was said in this issue thread, and on the 
python-dev mailing list (subject: == on object tests identity in 3.x), at least 
to the extent it was related to comparisons.

Besides the doc changes it contained previously, it now also contains 
improvements for the test suite for comparisons (lib/test/test_compare.py).

- Please review both.
 
Andy

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Added file: http://bugs.python.org/file35924/issue12067-expressions-py34_v8.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mark Dickinson]

Mark Dickinson added the comment:

+  In other words, the following expressions should have the same result:
+
+``x == y`` and ``not x != y``
+
+``x  y`` and ``not x = y``
+
+``x  y`` and ``not x = y``

I think the second and third items here go too far: sets don't obey these 
rules, for example.  Not all uses of comparisons need to force a total ordering.

OTOH, you leave out a more fundamental relation, namely that `x  y` and `y  
x` should ordinarily give the same result, as should `x = y` and `y = x`.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

I see.
But I don't think it is a sensible default, as the source code states.

The Python doc (v2 and v3) is quite consistent in stating that `==` compares 
the values of two objects, while `is` compares object identity.

Having a default implementation on the object type that implements `==` by 
comparing object identity is not consistent with that.

- Can someone please elaborate what the reason for that is?

- Where is the discrepancy between the documentation of == and its default 
implementation on object documented?

To me, a sensible default implementation for == on object would be (in Python):

  if v is w:
return True;
  elif type(v) != type(w):
return False
  else:
raise ValueError(Equality cannot be determined in default implementation)

Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Terry,
I'd like to comment on your statement:
 3. By default, == and /= compare identities.
in msg148774.

What experiment lead you to that conclusion?

Here is one that contradicts it (using cpython 3.4.1):

 i1 = 42
 f1 = 42.0
 i1 == f1
True
 i1 is f1
False

Is it possible, that your experiment got influenced by the optimization that 
attempts to reuse existing objects of immutable types?
Like in this:

 i1 = 42
 i2 = 40 + 2
 i1 == i2
True
 i1 is i2
True

Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploaded v5 of the patch.

Changes:

1. The statement that comparison of different built-in types (always) raises 
TypeError, was too general. Changed to distinguish equal and order operators, 
as summarized by Ezio in items 3) and 4) of msg148760.

2. Ensured max line length of 80, in text areas affected by the patch.

Andy

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Added file: http://bugs.python.org/file35850/issue12067-expressions-py34_v5.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

It seems I still need to practice creating patches ... uploading v6 which 
should create a review link. No other changes.
Sorry for that.
Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Another attempt. Really sorry...

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Added file: http://bugs.python.org/file35853/issue12067-expressions-py34_v7.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

Terry J. Reedy added the comment:

In py3, *everything* is an instance of class object. This makes like simple 
than in 2.x. The default comparison rules are set by the rich comparison 
methods of object. 'By experiment' meant by experiments with instances of 
object, which use those default methods, rather than by inspection of the 
relevant .c source code. Instances of subclasses taht do not override the 
defaults would act the same. Here is what seem to be the default code, from 
object.c, do_compare.  It verifies what I said (v, w are pointers, which 
represent identity):

/* If neither object implements it, provide a sensible default
   for == and !=, but raise an exception for ordering. */
switch (op) {
case Py_EQ:
res = (v == w) ? Py_True : Py_False;
break;
case Py_NE:
res = (v != w) ? Py_True : Py_False;
break;
default:
/* XXX Special-case None so it doesn't show as NoneType() */
PyErr_Format(PyExc_TypeError,
 unorderable types: %.100s() %s %.100s(),
 v-ob_type-tp_name,
 opstrings[op],
 w-ob_type-tp_name);
return NULL;
}
Py_INCREF(res);
return res;

Subclasses can and ofter do override the default methods. In particular, the 
number subclasses compare by value, across number types.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Hi, I'd like to revive this issue.

IMHO, the changes in issue12067-expressions_v2.diff go too far. I don't think 
that deleting the entire section about the details of comparing objects of the 
same type makes sense.

I agree with Terry's statement in msg170936 that the chapter is about the 
operators and not about the ways to customize them, so some of what the patch 
introduces in that area should not be introduced.

So far, that means that I'm pretty much against that patch entirely...

Having said that, I do believe that there are still issues:

1. both the 2.7 and 3.x sections about the comparison operators are 
sufficiently convoluted and could be organized better by grouping the various 
statements that are made, into categories like this:

- comparisons involving objects of user-defined types
- comparing objects of same built-in type
- comparing objects of differing built-in type

2. There are still some errors, ambiguities and omissions that need to be 
fixed. For example, in the 3.x version:

a) omission about treatment of NaN for numbers of different type (could in 
theory be implied from the statement are converted to same type, but that 
statement is problematic as was pointed out in Mark's comment on this issue).

b) Amgiguous statement Most numeric types [of same type] can be compared with 
one another.. I think what is true is that all built-in numeric types 
(including Fraction and Decimal) compare mathematically correct in 3.x, and 
that the only non-support is that complex numbers are not considered orderable 
and an attempt to use an ordering operator raises TypeError.

c) Ambiguous statement When cross-type comparison is not supported, the 
comparison method returns NotImplemented.. If this is about the customization 
methods, it should not be here, but there. Here, it is relevant that a 
TypeError is raised when using the operator.

d) Terminology in Bytes objects are compared lexicographically using the 
numeric values of their elements.: Chapter [4.8.1. Bytes] defines bytes 
objects as immutable sequences of single bytes (not elements).

e) Terminology in Tuples and lists are compared lexicographically using 
comparison of corresponding elements.: lists and tuples contain items not 
elements, and an item-wise comparison should not be called 
lexicographically because that makes sense only when the items are characters.

f) Ambiguity in If not equal, the sequences are ordered the same as their 
first differing elements.: Are ordered could be interpreted (e.g. by 
non-native speakers) to mean that the sequence is changed to achieve that 
ordering, which is not the case of course.

g) Editorial: In the list item about sets and froze sets, the example set {2,3} 
is not in example font.

h) Omission: Range types are not covered.

i) Omission: The section Comparison of objects of differing types... is 
silent about which built-in types support comparison across types (except for 
numeric types where that is covered). I think that should be explicitly listed 
for those built-in types that are also listed explicitly in the section about 
comparing objects of same type.

I'll try to come up with a patch for 3.x, and once that is agreed, with one for 
2.x.

Andy

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Changes by Andy Maier andreas.r.ma...@gmx.de:


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Added file: http://bugs.python.org/file35843/issue12067-expressions_v3.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploaded issue12067-expressions_v3.diff for the 3.5 tip.
Please review.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

Uploaded issue12067-expressions_v4.diff to improve the unicode footnote 3, and 
to revert to using the term lexicographical for sequences (after learning 
that it applies there as well). Also, this version was produced using hg diff 
to make it properly reviewable.

Please review.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Andy Maier]

Andy Maier added the comment:

PS: The v4 patch does not address comments f) and h) from msg04, and it 
seems to me they do not need to be addressed.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Mike Hoy added the comment:

Considering that the docs have changed does this issue still need to be open?

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Mike Hoy added the comment:

Changed patch to include suggestions by Chris Jerdonek.

http://bugs.python.org/issue12067#msg170953

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Added file: http://bugs.python.org/file27442/issue12067-expressions_v2.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Changes by Mike Hoy mho...@gmail.com:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Chris Jerdonek]

Chris Jerdonek added the comment:

Some minor comments:

-The operators , , ``==``, ``=``, ``=``, and ``!=`` compare the
+, , ``==``, ``=``, ``=``, and ``!=`` compare the values of two 

I think it reads better to start a sentence (and in this case a paragraph) with 
a word rather than a symbol.

-values of two objects.  The objects need not have the same type. If both are
+objects. The two objects don't necessarily have to be of the same type. With 

The replacement sentence seems wordier to me.

+(:meth:`__ge__()` reflects :meth:`__lt__()`, etcetera). Builtin numbers 
compare 
+as expected, even when of different types. Builtin sequences compare 

Built-in is hyphenated in the docs.  See, for example, here:

http://docs.python.org/dev/library/functions.html

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mark Dickinson]

Mark Dickinson added the comment:

 I determined that 'raise TypeError' and 'return NotImplemented' both
 result in the call of the reflected method

Are you sure?  raise TypeError *should* result in the operation being 
abandoned, with the reflected operation not tried.


Python 3.3.0rc2+ (default:3504cbb3e1d8, Sep 20 2012, 22:08:44) 
[GCC 4.2.1 (Apple Inc. build 5664)] on darwin
Type help, copyright, credits or license for more information.
 class A:
... def __add__(self, other):
... raise TypeError(Don't know how to add)
... def __le__(self, other):
... raise TypeError(Can't compare)
... 
[65945 refs]
 class B:
... def __radd__(self, other):
... return 42
... def __ge__(self, other):
... return False
... 
[66016 refs]
 A() = B()
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 5, in __le__
TypeError: Can't compare
[66064 refs]
 A() + B()
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 3, in __add__
TypeError: Don't know how to add
[66065 refs]

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

Terry J. Reedy added the comment:

You are right, I did not look deep enough. I was fooled by the conversion of 
NotImplemented, returned from object.__le__, etc, to TypeError. Sorry for that 
noise.

For comparison and arithmetic, the actual alternative to defining a function 
that returns NotImplemented seems to be to not define it at all.

class C():
def __ge__(self, other):
return True
def __add__(self, other):
return 44
__radd__ = __add__

class O():
pass  # removed NotImplemented defs

c = C()
o = O()
print(c = o, o = c)
# True True
print(c + o, o + c)
# 44 44

(I looked at the codes for binary_op1 in abstract.c and do_richcompare in 
object.c and do not yet see any effective difference between not defined and a 
NotImplemented return.)

I'll take a look at the patch later.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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type:  - enhancement

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Mike Hoy added the comment:

http://bugs.python.org/issue15997 is this issue related to what Terry has 
mentioned:

Does 'not supported' mean 'raises TypeError', 'returns NotImplemented', or 
both? If the last, I don't really understand the reason for NotImplemented 
versus TypeError. That point should be clarified in 3.3 also. And 3.3 should be 
referenced in the comparisons section.

I am working on this patch and need confirmation as to whether or not this has 
to be included in my patch. I'm not clear on it so I may just pass on making a 
patch if it is required for this issue.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

Terry J. Reedy added the comment:

After further thought: This section is about the syntax operators, not the 
special methods. The syntax operators never evaluate to NotImplemented as they 
(apparently) interpret its return from a special method the same as a raising 
of TypeError, and always raise TypeError when neither the op or its reflection 
is supported. So there should be no mention of NotImplemented here. Just a 
reference to 3.3. #15997 is related to my 'wonder' but not directly relevant to 
a patch for this. Please submit a draft patch when you have one.

I determined that 'raise TypeError' and 'return NotImplemented' both result in 
the call of the reflected method, at least for a couple of cases. (And same 
seems true for arithmetic ops too.)

class C():
def __ge__(self, other):
# print(in C.__ge__, end='')
return True
def __add__(self, other):
return 44
__radd__ = __add__

class O():
def __le__(self, other):
# print (in O.__le__)
return NotImplemented
def __add__(self, other):
return NotImplemented

c = C()
o = O()
ob = object() 
print(c = o, o = c, ob = c)
# True True True
# print(ob = ob) # raises TypeError
print(c + o, o + c, ob + c)
# 44 44 44
# print(ob + ob)  # raises TypeError
# print(ob = o)  # with O.__le__ print uncommented
# in O.__le__  # so non-implemented reflected o = ob *is* called
# TypeError: unorderable types: object() = O()

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Mike Hoy added the comment:

I've attempted to incorporate both Terry's and Ezio's suggestions. Here is a 
patch to get started with. There is a section that has been deleted. Patch 
uploaded.

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Added file: http://bugs.python.org/file27256/issue12067-expressions.diff

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mike Hoy]

Changes by Mike Hoy mho...@gmail.com:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Ezio Melotti]

Ezio Melotti ezio.melo...@gmail.com added the comment:

Would it be ok to state that:
1) , , ==, =, =, and != compare the values of two objects;
2) the two objects don't necessarily have to be of the same type;
3) with == and !=, objects of different types compare unequal, unless they 
define a specific __eq__ and/or __ne__;
4) with , , =, and =, the comparison of objects of different types raises a 
TypeError, unless they define specific __lt__, __gt__, __le__, and __ge__;
5) some built-in types define these operations, so it's possible to compare 
e.g. int and floats;

This should summarize the possible behaviors.  There's no reason IMHO to expose 
implementation details and to special case built-in types (unless their 
comparison is actually different and doesn't depend on __eq__, __ne__, etc.).

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

In Python 3, where all classes inherit from object, the default rules are, by 
experiment, (which someone can verify from the code) simpler than you stated.

3. By default, == and /= compare identities.
4. By default, order comparisons raise TypeError.
ob = ob raises even though ob == ob because ob is ob.

I am not sure of the method look-up rules for rich comparisons, but perhaps the 
following are true:

3) with == and !=, an object is equal to itself and different objects (a is not 
b) compare unequal, unless the class of the first define a specific __eq__ and 
__ne__;

4) with , , =, and =, comparison raises a TypeError, unless the class of 
the first object defines specific __lt__, __gt__, __le__, and __ge__, or the 
class of the second defines the reflected method (__ge__ reflects __lt__, 
etcetera);

What is not clear to me is whether the reflected method is called if the first 
raises TypeError. The special method names doc (reference 3.3) says A rich 
comparison method may return the singleton NotImplemented if it does not 
implement the operation for a given pair of arguments. 
...
There are no swapped-argument versions of these methods (to be used when the 
left argument does not support the operation but the right argument does); 
rather, __lt__() and __gt__() are each other’s reflection, __le__() and 
__ge__() are each other’s reflection, and __eq__() and __ne__() are their own 
reflection.

Does 'not supported' mean 'raises TypeError', 'returns NotImplemented', or 
both? If the last, I don't really understand the reason for NotImplemented 
versus TypeError. That point should be clarified in 3.3 also. And 3.3 should be 
referenced in the comparisons section.

I think point 5 should say a bit more: builtin numbers compare as expected, 
even when of different types; builtin sequences compare lexicographically.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

[Docs]
If both are numbers, they are converted to a common type.

[Terry]
In any case, I think it is only true for built-in number types,

It's not even true for built-in number types.  When comparing an int with a 
float, it's definitely *not* the case that the int is converted to a float and 
the floats compared.  And that's for good reason:  the int - float conversion 
is lossy for large integers, so if int - float comparisons just converted the 
int to a float before comparing, we'd have (for example):

 10**16 == 1e16 == 10**16 + 1

leading to broken transitivity of equality, and strange dict and set behaviour.

So int - float comparisons do a complicated dance under the hood to compare 
the exact numerical values of the two objects and produce the correct result.

I'm not sure what the intent of the original sentence was, or how to reword it. 
 The key point is simply that it *is* possible to compare an int with a float, 
and that the result is sensible, based on numeric values.

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Terry J. Reedy]

New submission from Terry J. Reedy tjre...@udel.edu:

Current 3.2 doc, 5.9. Comparisons, has this paragraph about mixed-type 
comparisons.

The operators , , ==, =, =, and != compare the values of two objects. The 
objects need not have the same type. If both are numbers, they are converted to 
a common type. Otherwise, the == and != operators *always* consider objects of 
different types to be unequal, while the , , = and = operators raise a 
TypeError when comparing objects of different types that do not implement these 
operators for the given pair of types. You can control comparison behavior of 
objects of non-built-in types by defining rich comparison methods like 
__gt__(), described in section Basic customization.

Sentence 3: If both are numbers, they are converted to a common type. I 
suspect it would be more true to say 'common internal type' as I would not 
think it a language requirement to produce Python objects.

In any case, I think it is only true for built-in number types, and I do not 
see that qualification anywhere previously.

That aside, it does not appear to be true for Decimals and Fractions in 2.7.1.

Sentence 4: first clause is only true for built-in types. That qualification is 
not obvious to everyone, as evidenced by a current python-list sub thread.

For 2.7, which has a different continuation, I suggest adding 'built-in' before 
'objects of'.
For 3.2/3, I suggest deleting '*always*' and adding a comma after 'TypeError' 
so that the 'when' condition applies to equality comparisons also.

After discussion about same-type comparisons, there is another paragraph about 
mixed-type comparison:

Comparison of objects of the differing types depends on whether either of the 
types provide explicit support for the comparison. Most numeric types can be 
compared with one another, but comparisons of float and Decimal are not 
supported to avoid the inevitable confusion arising from representation issues 
such as float('1.1') being inexactly represented and therefore not exactly 
equal to Decimal('1.1') which is. When cross-type comparison is not supported, 
the comparison method returns NotImplemented. This can create the illusion of 
non-transitivity between supported cross-type comparisons and unsupported 
comparisons. For example, Decimal(2) == 2 and 2 == float(2) but Decimal(2) != 
float(2).

I suggest deleting this entirely. The first sentence and first clause of the 
second repeat what was said above. The rest is obsolete as float/decimal 
comparisons *are* implemented in 2.7.1 and 3.2.0.

--
assignee: docs@python
components: Documentation
keywords: easy
messages: 135873
nosy: docs@python, rhettinger, terry.reedy
priority: normal
severity: normal
stage: needs patch
status: open
title: Doc: remove errors about mixed-type comparisons.
versions: Python 2.7, Python 3.2, Python 3.3

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[issue12067] Doc: remove errors about mixed-type comparisons.

[Chris Rebert]

Changes by Chris Rebert pyb...@rebertia.com:


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[issue12067] Doc: remove errors about mixed-type comparisons.

[Ezio Melotti]

Ezio Melotti ezio.melo...@gmail.com added the comment:

Can you provide a patch?

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