python
mypy is unable to correctly disambiguate a union when there is a dependent function type
Bug Report
mypy is unable to correctly disambiguate a union when there is a dependent function type.
The call meow(s)(s) is always valid as s is either int or str and should not be able to take on both in the line.
However, the dependency of the return type of meow(s) on s is not remembered and mypy becomes upset.
To Reproduce
https://mypy-play.net/?mypy=latest&python=3.11&gist=50bc3d85db8bc5120d881d37b891fd2a
from typing import overload, Callable, Union
@overload
def meow(a: int) -> Callable[[int], int]:
...
@overload
def meow(a: str) -> Callable[[str], str]:
...
def meow(a):
return lambda x: x
# OK
a: str = 's'
meow(a)(a)
# Not ok though it should be
s: Union[int, str] = 's'
meow(s)(s)
Expected Behavior
Expected Behavior
Success: no issues found in 1 source file
Actual Behavior
main.py:20: error: Argument 1 has incompatible type "Union[int, str]"; expected "int" [arg-type]
main.py:20: error: Argument 1 has incompatible type "Union[int, str]"; expected "str" [arg-type]
Found 2 errors in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 0.991
- Mypy command-line flags: none
- Mypy configuration options from
mypy.ini(and other config files): none - Python version used: 3.11
Hm, the problem is that mypy doesn't keep track of how the type of meow(s) was determined. It's just treated as a union type:
s: Union[int, str] = 's'
reveal_type(meow(s)) # Union[Callable[[int], int], Callable [[str], str]]
When you then do meow(s)(s), mypy only sees that meow(s)(x) either takes an int or a str but it's not known which one, so mypy throws an error.
I would predict that mypy will never support this but I can't be sure. In the meantime, you can maybe solve your problem with a generic function:
MyData = TypeVar("MyData", int, str)
def meow(a: MyData) -> Callable[[MyData], MyData]:
return lambda x: x
See also: https://github.com/python/mypy/issues/2008 (it's why mypy doesn't narrow s's type on assignment).
