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

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

def f(x: A): pass

y = A()

class A: pass

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

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

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.

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.

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

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

def foo():
    ...

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.

@ilinum

@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.

#14203 should fix this for classes and functions.

[use before def] handle class and function definitions (#14203)

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

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References to names before they're bound should generate an error (UnboundLocalError) #686

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References to names before they're bound should generate an error (UnboundLocalError) #686

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spkersten commented on May 18, 2015

JukkaL commented on May 19, 2015

carljm commented on Mar 8, 2018

JukkaL commented on May 17, 2018

DetachHead commented on Mar 21, 2022

PeterJCLaw commented on Nov 3, 2022

ilevkivskyi commented on Nov 26, 2022

ilinum commented on Nov 27, 2022

PeterJCLaw commented on Nov 29, 2022

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