Pyflakes

Undefined variable defined later in a loop

Bug #1686327 reported by asmeurer on 2017-04-26

This bug affects 1 person

Affects		Status	Importance	Assigned to	Milestone
	Pyflakes	Won't Fix	Undecided	Unassigned

Bug Description

For the following code

def test():
    a = False
    for i in range(10):
        if a:
            print(b)
        else:
            a = True
            b = 1

pyflakes gives the following errors

test.py:5: undefined name 'b'
test.py:8: local variable 'b' is assigned to but never used

But test() runs just fine without any errors.

Revision history for this message

Phil Frost (bitglue) wrote on 2017-04-29:

Here we know a is False because it was assigned with a literal. But what if it's the result of a function call, or even a more complex expression?

Have you encountered this in the wild?

Revision history for this message

asmeurer (asmeurer) wrote on 2017-04-30:

I hit it in my own code. But, honestly, it's a very unstable coding pattern. I ended up adding a minor thing to it which required defining the variable before the loop. So if fixing this is too hard or adds too many false negatives I wouldn't be too worried about it.

Revision history for this message

Phil Frost (bitglue) wrote on 2017-05-11:

Yeah, I think it would be pretty tricky. Right now pyflakes skips over conditional branches as if they aren't even there, so all it sees is:

    a = False
    print(b)
    a = True
    b = 1

Technically it could be fixed when `a` is assigned a literal, but between the effort required to implement the fix and the added complexity to the code I don't think it's worthwhile.

Changed in pyflakes:
status:	New → Won't Fix

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