[code-quality] Warn on list comprehension variables used outside list comprehensions?

[Phil Frost]

On Wed, Nov 26, 2014 at 12:41 AM, <code-quality.wting at xoxy.net> wrote:

> Python binds list comprehension variables to the local scope which has
> caused some subtle bugs. Is it possible to add a warning for this in
> pyflakes?
>

 A guiding principle of pyflakes's design is that it doesn't have warnings.
If it complains about anything, it is because it's almost certainly useless
or broken code. Using a list comprehension variable outside of the
comprehension is perfectly valid according to python2:

>>> [x for x in range(3)]
[0, 1, 2]
>>> x
2

but an error in python3, which has different scoping rules:

>>> [x for x in range(3)]
[0, 1, 2]
>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

So in the case of python3, it's a bug that it doesn't emit a warning for
this code. For python2, pyflakes is working as designed.

There's an issue, though. Pyflakes doesn't know if it's checking python2 or
python3 code. It uses the AST of whatever python is running pyflakes, and
if you look around checker.py[1] you can see there are some conditionals on
PY2, PY32 and PY33. I think though all of these are to accommodate
differences in the AST, but not to alter the checking semantics.

Given that parsing python2 with a python3 AST is already going to be
problematic for a lot of other reasons (like `print` vs `print()`), maybe
the thing to do is say that pyflakes always checks input against the
semantics of the version of python running pyflakes. Then implement some
tests, and change how list comprehensions are handled[2] to be conditional
on the version. I think everyone using pyflakes for python2 and python3
concurrently either has it installed twice or invokes it with the
interpreter corresponding to what they want to check, ie `python2 -m
pyflakes` or `python3 -m pyflakes`, so I can't think of any reason this
wouldn't work, and then pyflakes can continue not having any configuration
file or options. Pull requests welcome.

  [1]: https://github.com/pyflakes/pyflakes/blob/master/pyflakes/checker.py
  [2]:
https://github.com/pyflakes/pyflakes/blob/master/pyflakes/checker.py#L657
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