[Python-ideas] Order of loops in list comprehension

[C Anthony Risinger]

On Oct 22, 2016 2:51 AM, "Alexander Heger" <python at 2sn.net> 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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