Erroneous 'list comprehension redefines' in Python 2.7 pyflakes 0.6.1

[Jayson Falkner]

When updating from pyflakes 0.5.0 to pyflakes 0.6.1 many occurances of 'list comprehension redefines' are listed erroneously. The issue appears to be that pyflakes will complain about reusing the same loop variable name in multiple list comprehensions.

Here are two lines for example:

168:
wells = Well.objects.filter(id__in=(well.id for well in wells))

...

183:
fx_wells = [well for well in wells if well.assay_subtype == fx_subtype]

And a message such as below appears.

foo/bar.py:183:: list comprehension redefines 'well' from line 168

In this case pyflakes should be smart enough to realize that 'well' will not be incorrectly used because it will be initialized immediately with the expected value in the second list comprehension.

[Ian Cordasco (icordasc)]

I couldn't reproduce this with the attached file.

[James (bistromathic1)]

This problem seems to occur when a dummy variable name in a generator expression, a dict comprehension, or a set comprehension is then reused in a list comprehension, for example:

a = [1, 2, 3, 4, 5, 6]

b = (i + 1 for i in a)
c = [i + 2 for i in b]

print(c)

The pyflakes output is:

test-1.py:4: list comprehension redefines 'i' from line 3

I only tried this in python 3.2.

[Florent (florent.x)]

actually, this should not report redefinition of variable for:
 * generator expressions
 * dict comprehensions
 * set comprehensions

Only the list comprehension leaks its variable, and only with Python 2.x.
(Though we'll keep reporting the warning for 3.x as well)

[Florent (florent.x)]

This should fix it.

https://code.launchpad.net/~pyflakes-dev/pyflakes/1112067-comprehension-scope/+merge/147264

[Dan Kenigsberg (danken)]

I'm not sure it's the very same bug, but pylakes 0.6.1 (python 2.7.3) considers the following as having a redefinition of x:

if False:
    x = 1
else:
    [x for x in [1]]

[Florent (florent.x)]

> I'm not sure it's the very same bug, but pylakes 0.6.1 (python 2.7.3) considers the following as having a redefinition of x:
>
> if False:
> x = 1
> else:
> [x for x in [1]]

this last one is a different bug, it should be reported separately.

[Florent (florent.x)]

The fix for this second bug was added to the same branch:
https://code.launchpad.net/~pyflakes-dev/pyflakes/1112067-comprehension-scope/+merge/147264