Mypy issue with python scoping rule

[h8hawk]

Mypy pass following code as valid:

def now() -> str:
    return "now"


def usage() -> None:
    now = now()
    print(now + "!")


usage()

Snippet will raise UnboundLocalError error.

[JukkaL]

It would be nice if mypy would catch this error.

[ehudhala]

Hey, I started working on an implementation but ran into many edge cases.
After researching, it seems this behavior has been discussed previously and it was decided not to implement it (#2400 ).

I would like to add that semantic analysis currently doesn't know where in the scope a symbol is defined, so even a simple case like the following passes mypy successfully:

def func() -> None:
    assigned = unassigned + 1
    unassigned: int = 0

Another interesting behavior is that it confuses the type-checker somewhere down the road (I haven't looked into that yet) and it can't determine the type of an unassigned variable, so the following error is produced:

def func() -> None:
    assigned = unassigned + 1
    unassigned = 0
[out]
a.py:2: error: Cannot determine type of 'unassigned'

@JukkaL - how would you suggest we proceed from here?