False positive undefined name after del in branch

[Dima Tisnek]

Consider this test code:

def foo():
    bar = 1
    if 0:
        del bar
    else:
        del bar

pyflakes reports test.py:6: undefined name 'bar', that is 2nd `del bar` is considered in error.

understandably it is impossible to analyse all code paths in general case, so how about tagging `bar` as `uncertain if label is present` and allowing to `del` it?

[Luciano Ramalho (ramalho)]

Here is another false positive, for a not-so-subtle case:

Using flake8, which uses PyFlakes, I get the report:

board.py:14:19: F821 undefined name 'StandardError'

However, StandardError is a built-in exception in Python 2 (but not in Python 3).

[Florent (florent.x)]

I agree with the initial bug reported : probably we could do better in this case.

The issue reported by @ramalho is not related to the initial report, and it's not an issue at all:

    $ python3 -m pyflakes <<< StandardError
    <stdin>:1: undefined name 'StandardError'
    $ python2 -m pyflakes <<< StandardError
    $

[Daniel Hahler (blueyed)]

I came across a similar issue like in comment #1, where flake8/pyflakes on python3 reports "F821: undefined name 'unicode'" , but the code path is not triggered for Python3:

    if sys.version_info < (3, 0):
        def __unicode__(self):
            return self.url

        def __add__(self, other):
            return unicode(self) + other
    else:
        def __str__(self):
            return self.url

        def __add__(self, other):
            return str(self) + other

[John Vandenberg (jayvdb)]

This bug is about `del` (and the cases in https://bugs.launchpad.net/pyflakes/+bug/1431099 appear to be the same problem)