References to names before they're bound should generate an error (UnboundLocalError)

[spkersten]

This code raises an error when executed but passes type checking:

def f(x: A): pass

y = A()

class A: pass

[spkersten]

I'm working on a solution for this issue in my 'scope' branch.

[JukkaL]

Yup, mypy is currently too lenient, and catching these errors would be useful.

This should still be fine and we shouldn't complain about this:

def f(x: 'A'):
    y = A()

class A: pass

[carljm]

In Python 3.7, with from __future__ import annotations, the original example here is fine at runtime and should not raise an error in mypy either. See https://www.python.org/dev/peps/pep-0563/

Given that from __future__ import annotations is supposed to become the default behavior over time, perhaps implementing this form of strictness is not a high priority.

[JukkaL]

This is confusing for new users and they might not know about from __future__ import annotations so it would still be nice to generate better error messages.

[DetachHead]

another example that from __future__ import annotations doesn't fix:

foo() # runtime NameError: name 'foo' is not defined

def foo():
    ...

[PeterJCLaw]

Should we expect mypy to catch cases where the name could have been defined, such as in a try...except? IMO it would be great if it could, though I realise it's a bit more involved than the other cases described here:

try:
   raise ValueError
   bar = 3
except Exception:
   print(bar)

Obviously this code actually produces NameError rather than UnboundLocalError but the idea is the same.

[ilevkivskyi]

@ilinum I think the original example in this issue should result in use-before-def for the y = A() line, but it looks like it doesn't. Are you specifically excluding references to types? I think we should only allow forward references to types in type context (e.g. rvalue of a type alias definition), but not in runtime context.

[ilinum]

#14203 should fix this for classes and functions.

[PeterJCLaw]

Connecting the dots: looks like #4019 tracks the local variable case.