Re: [Python-ideas] Order of loops in list comprehension

[Greg Ewing]

C Anthony Risinger wrote:
Erlang/Elixir (sorry after 6 years python this is what I do now!) 
does it the same way as python:


 >>> [{X, Y} || X <- [1,2,3], Y <- [a,b]].
[{1,a},{1,b},{2,a},{2,b},{3,a},{3,b}]

Here X is the outer loop.

I think the confusion stems from doing it both ways at the same time.


If the semicolon syntax I suggested were available,
you'd be able to choose either order, and maybe even
mix them in the one comprehension. Not sure if that's
a good thing or not...

--
Greg
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Re: [Python-ideas] Order of loops in list comprehension

[C Anthony Risinger]

On Oct 22, 2016 2:51 AM, "Alexander Heger"  wrote:
>>>
>>>
 For me the current behaviour does not seem unreasonable as it
resembles the order in which you write out loops outside a comprehension
>>>
>>>
>>> That's true, but the main reason for having comprehensions
>>> syntax in the first place is so that it can be read
>>> declaratively -- as a description of the list you want,
>>> rather than a step-by-step sequence of instructions for
>>> building it up.
>>>
>>> If you have to stop and mentally transform it into nested
>>> for-statements, that very purpose is undermined.
>>
>> Exactly.
>
>
> Well, an argument that was often brought up on this forum is that Python
should do things consistently, and not in one way in one place and in
another way in another place, for the same thing.  Here it is about the
order of loop execution.  The current behaviour in comprehension is that is
ts being done the same way as in nested for loops.  Which is easy enough to
remember.  Same way, everywhere.

A strict interpretation by this logic would also require the [x ...] part
to be at the end, like [... x] since that's how it would look in a nested
for loop (inside deepest loop).

I personally agree with what many others have said, in that comprehension
order is not intuitive as is. I still page fault about it after many years
of using.

Is there a way to move the expression bit to the end in a backcompat way?
It might be a completely different syntax though (I think both colons and
semicolons were suggested).

FWIW, Erlang/Elixir (sorry after 6 years python this is what I do now!)
does it the same way as python:

>>> [{X, Y} || X <- [1,2,3], Y <- [a,b]].
[{1,a},{1,b},{2,a},{2,b},{3,a},{3,b}]

Here X is the outer loop.

I think the confusion stems from doing it both ways at the same time. We
retain the for loop order but then hoist the expression to the top. Ideally
we'd either not do that, or reverse the for loop order.
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Re: [Python-ideas] Order of loops in list comprehension

[Sven R. Kunze]

On 22.10.2016 09:50, Alexander Heger wrote:
Well, an argument that was often brought up on this forum is that 
Python should do things consistently, and not in one way in one place 
and in another way in another place, for the same thing.


Like * in list displays? ;-)

Here it is about the order of loop execution.  The current behaviour 
in comprehension is that is ts being done the same way as in nested 
for loops.


It still would. Just read it from right to left. The order stays the same.


Which is easy enough to remember.  Same way, everywhere.


I am sorry but many disagree with you on this thread.

I still don't understand why the order needs to be one-way anyway. 
Comprehensions are a declarative construct, so it should be possible to 
mix those "for .. in .."s up in an arbitrary order.


Cheers,
Sven
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Re: [Python-ideas] Order of loops in list comprehension

[Sven R. Kunze]

On 21.10.2016 01:43, Greg Ewing wrote:

Alexander Heger wrote:
For me the current behaviour does not seem unreasonable as it 
resembles the order in which you write out loops outside a comprehension


That's true, but the main reason for having comprehensions
syntax in the first place is so that it can be read
declaratively -- as a description of the list you want,
rather than a step-by-step sequence of instructions for
building it up.

If you have to stop and mentally transform it into nested
for-statements, that very purpose is undermined.



Exactly.


Sven
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Re: [Python-ideas] Order of loops in list comprehension

[Greg Ewing]

Alexander Heger wrote:
For me the current behaviour does not seem unreasonable as it resembles 
the order in which you write out loops outside a comprehension


That's true, but the main reason for having comprehensions
syntax in the first place is so that it can be read
declaratively -- as a description of the list you want,
rather than a step-by-step sequence of instructions for
building it up.

If you have to stop and mentally transform it into nested
for-statements, that very purpose is undermined.

--
Greg
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Re: [Python-ideas] Order of loops in list comprehension

[Alexander Heger]

For me the current behaviour does not seem unreasonable as it resembles the
order in which you write out loops outside a comprehension except that the
expression for generated values is provided first.

On 21 October 2016 at 05:03, Sven R. Kunze  wrote:

> On 19.10.2016 00:08, Rob Cliffe wrote:
>
>>
>> But it's far too late to change it now, sadly.
>>>
>> Indeed. :-(  But if I were ruler of the world and could have my own
>> wish-list for Python 4, this (as per the first example) would be on it.
>>
>
> I don't see no reason why we can't make it.
>
> Personally, I also dislike this behavior.
>
>
> Cheers,
> Sven
>
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Re: [Python-ideas] Order of loops in list comprehension

[Sven R. Kunze]

On 19.10.2016 00:08, Rob Cliffe wrote:



But it's far too late to change it now, sadly.
Indeed. :-(  But if I were ruler of the world and could have my own 
wish-list for Python 4, this (as per the first example) would be on it.


I don't see no reason why we can't make it.

Personally, I also dislike this behavior.


Cheers,
Sven
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Re: [Python-ideas] Order of loops in list comprehension

[Rob Cliffe]

On 18/10/2016 07:40, Greg Ewing wrote:

Random832 wrote:

For me, it's the fact that:
[[a for a in b] for b in ['uvw', 'xyz']] == [['u', 'v', 'w'], ['x', 'y',
'z']]
which makes me want to write:
[a for a in b for b in ['uvw', 'xyz']]


You're not alone! Lately I've been becoming convinced that
this is the way we should have done it right back at the
beginning.
Me too.  When I first got to grips with the order of loops in a list 
comprehension, I found it counter-intuitive and jarring.  I still have 
to remind myself each time.
I guess different people have different mental models, and may feel 
differently.  The best way I can think of to explain why I feel this way is:


If the syntax were
[ x for x in alist for alist in list-of-lists ]
there is a smooth (or rather, monotonic) gradation from the smallest 
object (x) to the next biggest object (alist) to the biggest object 
(list-of-lists), which IMHO is easier to follow.  Each object is 
conceptually zero or one steps from its neighbour.

But with the actual syntax
[ x for alist in list-of-lists for x in alist ]
there is a conceptual hiatus after "x" ("what on earth are alist and 
list-of-lists, and what have they got do to with x?").  This would be 
even more marked with more than 2 loops: we jump from the lowest level 
object to the two highest level objects, and it all seems like a 
disorienting non-sequitur until the very last loop "joins the dots".  
You have to identify the "x" at the beginning with the "x" near (but not 
at!) the end.  Instead of (ideally, if not always in practice) reading 
the expression from left-to-right in one go, your eyes are forced to 
jump around in order for your brain to assimilate it.


A possible alternative syntax might be to follow more closely the 
for-loop syntax, e.g.

[ for alist in list-of-lists: for x in alist: x ]
Here the "conceptual jump" between each object and the next is either 1 
or 2, which for me makes this a "second best" option.  But at least the 
"conceptual jump" is bounded (regardless of the number of loops), and 
this syntax has the advantage of familiarity.



But it's far too late to change it now, sadly.
Indeed. :-(  But if I were ruler of the world and could have my own 
wish-list for Python 4, this (as per the first example) would be on it.


Best wishes
Rob Cliffe
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