False negative undefined-variable after a caught exception shadowing a variable name

[pylint-bot]

Originally reported by: BitBucket: moxian, GitHub: @moxian?

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Pylint'ing snippet

def main():
    exc = None
    try:
        [][1] = 12
    except IndexError as exc:
        pass
    if exc is not None:
        raise exc


main()

says

Raising NoneType while only classes, instances or string are allowed (raising-bad-type)

when in fact None will never be attempted to be raised

Python 2

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• Bitbucket: https://bitbucket.org/logilab/pylint/issue/157

[pylint-bot]

Original comment by Sylvain Thénault (BitBucket: sthenault, GitHub: @sthenault?):

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pylint has a somewhat dumb static inference system which doesn't handle flow handling. Also in your case I suspect the code could be rewritten in a way both understandable by pylint and more readable, but we would need more context for that.

[Pierre-Sassoulas]

Given:

# pylint: disable=missing-docstring


def main():
    exc = None
    print(exc)
    try:
        [][1] = 12
    except IndexError as exc:
        print(exc)
    if exc is not None:
        raise exc


main()

Pylint does not raise any errors but running the code gives:

None
list assignment index out of range
Traceback (most recent call last):
  File "a.py", line 15, in <module>
    main()
  File "a.py", line 11, in main
    if exc is not None:
UnboundLocalError: local variable 'exc' referenced before assignment

So it actually became a false negative for undefined variable in python 3.