Using possibly undefined loop variable

[chabotsi]

In the code below, the variable value is possibly undefined. It would be nice to have a warning. For instance, Pylint gives the following one « W0631: Using possibly undefined loop variable 'value' (undefined-loop-variable) ».

#!/usr/bin/env python3
# coding: utf-8

"""
    the variable `value` is possibly undefined.
"""


def print_content(iterable):
    """
        print the content of the given iterable
    """
    for value in iterable:
        print(value)
    last_value = value
    print('The last_value is ', last_value)


print_content('abcd')
print_content([])

[sigmavirus24]

So to be clear, if you run this on python 3 as is, you should get:

UnboundLocalError: local variable 'value' referenced before assignment

But this is not an issue with Python 2.

Also if you add something like:

    value = None

Prior to the

    for value in iterable:

Then this doesn't error. As long as the context for how a variable is created is preserved (created as part of a for-loop for example, or on Python 3.8 with :=) then this would be possible. I suspect, however, that Pyflakes method for finding/tracking bindings is too simplistic for this to work.

[chabotsi]

But this is not an issue with Python 2

I obtain the same error in python2.

Also if you add something like:
value = None

Sure, the whole point is that we should get a warning to add this declaration prior to the loop. No ?

[bitglue]

Pyflakes should report this as an error in python3, but in my testing (2.1.1 Python 3.7.2 on Darwin) it does not. I think this may be a regression.

In python2 there won't be an error because value might be defined.

[asottile]

collecting duplicates for "pyflakes doesn't do branch analysis" here: #715