False-negative used-before-assignment after try-except block

[cdce8p]

Followup to #5500 (comment)

def func(var):
    try:
        return 1 / var.some_other_func()
    except AttributeError:
        pass
    except ZeroDivisionError:
        msg = "Devision by 0"

    print(msg)  # should emit 'used-before-assignment'

The code above should raise a used-before-assignment error since msg is not assigned in the AttributeError ExceptHandler.

[jacobtylerwalls]

Extending this example, if the try block actually defines msg instead of returning, since there is not exhaustive error handling (by assigning the name, or raising or exiting), then we could emit a message that the use of the name needs to be guarded by else. Maybe that's also used-before-assignment, or maybe it's not. 🤔

Maybe consider-using-try-else (or if that's too weasel-worded, maybe nonexhaustive-error-handling?)

try:
    a = 1 / n
except LibraryError:
    pass
print(a)  # [nonexhaustive-error-handling]

try:
    a = 1 / n
except LibraryError:
    pass
else:
    print(a)  # no message

From discussion in #5582 (comment).

[jacobtylerwalls]

We can close #2835 as a duplicate.

[cdce8p]

Another idea. I like how pyright names it. Maybe possibly-unbound?

"a" is possibly unbound

Maybe consider-using-try-else

We shouldn't really "force" users to add an else case. There are just to many alternatives. You could for example assign a default value before you enter the try block.

[DanielNoord]

Some of the discussion in this issue is being moved to #2835. The rest is being closed with #5764.