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

[Ian Cordasco]

On Wed, Nov 26, 2014 at 9:27 AM, Keith Derrick <keith.derrick at lge.com> wrote:
> Definitely seems like something that should be warned about.
>
> For python2 it ought to be an error (which can always be turned off) for
> either code snippet.

PyFlakes doesn't allow you to ignore errors. Flake8 does so this is a
moot point.

> For python3, I'd say the two line snippet should flag an error - yes, it
> will fail at run-time, but pydev in eclipse runs pylint and saves me a
> lot of grief by spotting this type of thing before I ever save the file
> (though it doesn't catch either of these, presumably because they are
> valid python).

If PyLint doesn't catch this I wonder if it's either never been raised
or been decided to be a bad idea. Could someone search their bugs?

> The three line snippet should probably give a warning ("Are you sure you
> mean this?")
>
> The question is, how do we know which version of Python the code under
> inspection is targeted for, and hence how to treat this case.
>
> Some thoughts on that:
>
> * Anyone serious enough to be using pyflakes, pylint, or other static
> analysis tools is also probably using virtualenv.
> * pyflakes is therefore most likely running under the same python
> version for which the code is targeted.
> * code which is meant to run under multiple python versions is probably
> tested under something like tox - so again, pyflakes is run under the
> targeted python version (certainly my tox.ini runs it in every target
> environment)
>
> There's a lot of "probably"'s in there, but I don't think they are
> unreasonable assumptions.
>
> So, could  pyflakes reasonably assume that the interpreter under which
> it is running is also the intended target version for the code under
> inspection? At least for the major version.

It already works this way. If you use pyflakes on Python 2 but are
writing code only valid in Python 3 you will have a bad time. You have
to install pyflakes on the version of Python you want it to lint for
since pyflakes works only with the in-built ast module of Python.

> This could then be overridden by a suitable command line option to
> specify the target version(s)

No. This isn't how PyFlakes is intended to work and this is extraneous.

> Also, in cases like this, possibly a "This will behave differently in
> Python 2 and 3" warning is a worth while alternative.

Again, PyFlakes is very different from PyLint. We don't issue warnings
like this.

> Keith Derrick | Principal Engineer, Connected Platform | Engineering
> LG Silicon Valley Lab | 5150 Gt America Parkway, Santa Clara, CA 95054
> Office: 408.610-5746 | Mobile: 831.383.9567 |  LG.com
>
> On 11/26/2014 06:57 AM, Skip Montanaro wrote:
>>> That's not the point of this check. The point of this check is that on
>>> Python 2, the binding to x in the comprehension bleeds outside of the
>>> comprehension scope
>> Got it.
>>
>> Still, this code:
>>
>>>      x = 10
>>>      [x for x in range(3)]
>>>      print(x + 1)
>> will run differently in Python 2 than Python3, so even if that was a
>> conscious choice by the author, a --py3k flag should cause a message
>> for this code. This is even worse than the simpler
>>
>>>      [x for x in range(3)]
>>>      print(x + 1)
>> because at least that will raise a NameError when run in Python 3. The
>> three-line construct will still run, though produce different output.
>>
>> Skip
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