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MyPy forgets callable type guard in lambda in return statement of function with hinted return type #15862
Description
Bug Report
There seems to be some strange interaction between hinted return types, lambdas and the callable type guard.
To Reproduce
Here is a minimal example, together with some related examples that are not broken, showing that the return type, lambda and callable are all required for this to break:
(Playground: https://mypy-play.net/?mypy=latest&python=3.11&gist=26f7a80bf3eff44ebf560ce5db694107)
from typing import Callable
def foo(x: int | Callable) -> int | Callable:
if callable(x):
return lambda: x()
else:
return x
def foo_fixed(x: int | Callable) -> int | Callable:
if callable(x):
ret = lambda: x()
return ret
else:
return x
def foo_fixed2(x: int | Callable):
if callable(x):
return lambda: x()
else:
return x
def foo_fixed3(x: int | float) -> int | Callable[[], float]:
if isinstance(x, float):
return lambda: x
else:
return x
def foo_fixed4(x: int | Callable) -> float | Callable:
if callable(x):
return x
else:
return x + 1.0Expected Behavior
Mypy should remember that x is a Callable within the lambda in the return statement.
Actual Behavior
MyPy forgets the type guard, and thinks that x is int | Callable in line 5:
main.py:5: error: "int" not callable [operator]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.5.0
- Mypy command-line flags: -
- Mypy configuration options from
mypy.ini(and other config files): - - Python version used: 3.11
Activity
hauntsaninja commentedon Aug 13, 2023
Thanks for the issue!
The underlying reason is that closures in Python are late binding, see https://mypy.readthedocs.io/en/stable/common_issues.html#narrowing-and-inner-functions.
mypy recently improved heuristics here (see #2608), but I guess not quite well enough to avoid the false positive in your case.
hauntsaninja commentedon Jan 4, 2024
Fixed by #16407