Use of undefined variable causes unrelated local class to leak scope

[denballakh]

Bug Report
I noticed a strange behavior and inconsistency when first argument of typing_extensions.assert_type is not defined.

To Reproduce
First case:

## a.py
def f() -> None:
 from typing_extensions import assert_type

 class X:
 pass

 # x = X() # note this line is commented out
 assert_type(x, X)


## b.py
from a import X
reveal_type(X)
# Revealed type is "def () -> a.X@5"

Second case:

## a.py
def f() -> None:
 from typing_extensions import assert_type

 class X:
 pass

 x = X() # note this line is not commented out
 assert_type(x, X)


## b.py
from a import X
reveal_type(X)
# Module "a" has no attribute "X"
# Revealed type is "Any"

To reproduce run mypy b.py.
This snippets differ by only one line x = X(), but mypy behavior is very different. It gives me module has no attribute error, or gives no error.

Expected Behavior
I expect to get the Module "a" has no attribute "X" error in both cases

My Environment

• mypy 0.961 (compiled: no)
• no command-line flags, no config file
• CPython 3.10.4
• Windows 10

[denballakh]

Another reproducer:

## a.py
def f() -> None:
 from typing_extensions import assert_type

 # first case: X is importable from this file
 class X:
 a: A # error: name "A" is not defined

 # second case: X is **not** importable from this file
 # class X:
 # pass

 x = X()
 assert_type(x, X)


## b.py
from a import X
reveal_type(X)

# in this file i get:
# in first case:
# note: Revealed type is "def () -> a.X@5"

# in second case:
# error:Module "a" has no attribute "X"
# note: Revealed type is "Any"

[hauntsaninja]

Interesting, thanks for finding this. Note that it also repros with any use of x, not just assert_type

E.g.:

a.py

def f() -> None:
 class X:
 ...
 x

b.py

from a import X
reveal_type(X)

[denballakh]

Other reproducer, but without using undefined variables:

## a.py
from __future__ import annotations
def f() -> None:
 class X:
 z: Z # i guess, mypy thinks Z isnt defined and leaks all classes into global scope, but there are no errors
 class Z: ...
 a: int
 t: type[X]

## b.py
from a import X, Z, a, t
# Module "a" has no attribute "a" # ok
# Module "a" has no attribute "t" # ok
reveal_type(X)
# Revealed type is "def () -> a.X@3" # not ok
reveal_type(Z)
# Revealed type is "def () -> a.Z@6" # not ok

Note that it leaks only classes, not variables.
If i change order of definition of X and Z everything works as expected.

[hauntsaninja]

Took a look at what's going on here, looks like there's some pretty cursed stuff happening in

mypy/mypy/semanal.py

Line 1449 in 0054046

def prepare_class_def(self, defn: ClassDef, info: Optional[TypeInfo] = None) -> None:

Also see #6422