False positive undefined name, which is defined in branch

Pyflakes ignores conditional branches, since short of being a Python interpreter it can't really know if they will branch or not. So all pyflakes sees is this:

j += 1
j = 0
print(j)

j = 0
j += 1
print(j)

Can you think of a way to handle this situation which doesn't involve running the code, and which does not generate false positives?

I think false positives come from names that aren't defined in conditionals that are used afterwords, like

if something:
    i = 0
else:
    # i not defined
if something:
    f(i)
else:
    # code that doesn't use i

If Pyflakes warned that the first else block didn't define i it would be a false positive, because i is never accessed when the code reaches that control flow.

But in this case, the variable is not defined *in* the conditional block. I don't see how that could be a false positive. So it's a distinction between

- Variables defined in only a subset of conditionals, and used below (fine)
- Variables used in a conditional that aren't defined above (not fine)

In a loop body, conditionals below does not always run before conditionals above

for example: statement_2 does not necessarily run after statement_1

while condition_1:
    if condition_2:
        statement_1
    else:
        statement_2

@bitglue
I think treating all variables defined in other branches as defined can silence the warning.
I have no opinions on whether doing this can be justified by the reported case, though.

@asmeurer
I said false positive because Pyflakes warned of code that can be run in the python interpreter. Maybe I have used the term incorrectly.

Also, at least to me, the problem seems to be more clear now. I can update the original report if needed.

Edit:
In a loop body, conditionals below does not always run before conditionals above
Should be
In a loop body, conditionals below does not always run *after* conditionals above