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Mypy reports error for different argument, than the one which is actually wrong #19289
Description
Bug Report
When using generic Callables as parameters, the error is correctly reported, but it's reported for different argument than the one which is in fact wrong.
To Reproduce
from typing import Callable, TypeVar, List
T1 = TypeVar('T1')
T2 = TypeVar('T2')
def bigfunc(
a: int,
b: T1,
c: Callable[[T1], T2],
d: Callable[[T2], str],
) -> None:
res = d(c(b))
print(f'{a=}, {b=}, {res=}')
# should be:
# c_func(x: float) -> List[float]:
def c_func(x: float) -> None:
pass
def d_func(y: List[float]) -> str:
return str(y)
bigfunc(1, 0.1, c_func, d_func)Expected Behavior
I would expect mypy to complain about 3rd argument (c - c_func).
Actual Behavior
Mypy complains about 1st argument, which is 100% correct (a: int)
a.py:24: error: Cannot infer type argument 1 of "bigfunc" [misc]
Found 1 error in 1 file (checked 1 source file)
Your Environment
- Mypy version used: 1.16.0
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): None - Python version used: 3.11.2
Activity
brianschubert commentedon Jun 13, 2025
To clarify, it's complaining about the first type argument, not the type of the first argument.
bigfunctakes two type arguments, one for each ofT1andT2. The error is stating that it failed to infer a value for the first one,T1.However, that's still wrong. It actually infers
T1fine and wittles the signature ofbigfuncdown tomypy then fails to infer a value for
T2, which it sees as being the first type parameter of the refined signature.sterliakov commentedon Jun 13, 2025
Can we just use tvar name instead of index? Names don't change when some generics are substituted, and are no less helpful for identification.
jan-spurny commentedon Jun 13, 2025
@sterliakov that would be great!
Show name of type variable in "Cannot infer type argument" message (p…