Regression since mypy 1.7 with functions that return a generic protocol
Open
Nobody has claimed this yet.
bug
- Dominant language
- Python
- Stars
- 20.6k
- Forks
- 3.3k
- PR merge metrics
- PR metrics pending
Description
EDIT: I found a simpler example reproducing the regression, see below
The following code used to pass with mypy 1.6.1
from typing import (
Protocol,
TypeVar,
Callable,
Concatenate,
ParamSpec,
reveal_type,
)
X = TypeVar("X")
Y = TypeVar("Y")
P = ParamSpec("P")
I = TypeVar("I", contravariant=True)
O = TypeVar("O", covariant=True)
# A callable converting I to O with parameters P
ConvertorCallable = Callable[Concatenate[I, P], O]
# An object that can convert I to O with parameters P
class ConvertorProtocol(Protocol[I, P, O]):
def convert(self, __source: I, *args: P.args, **kwargs: P.kwargs) -> O:
...
# Decorator to convert a callable to a convertor
def convertor(
func: ConvertorCallable[X, P, Y],
) -> ConvertorProtocol[X, P, Y]:
class Convertor:
def convert(self, source: X, /, *args: P.args, **kwargs: P.kwargs) -> Y:
return func(source, *args, **kwargs)
return Convertor()
# A convertor that converts X to a list of X
@convertor
def as_list(source: X, repeat: int = 1) -> list[X]:
return [source] * repeat
if __name__ == "__main__":
result = as_list.convert(1, repeat=3)
reveal_type(result)
print(result)
However, it no longer passes with mypy 1.7.0 and later:
error: Argument 1 to "convert" of "ConvertorProtocol" has incompatible type "int"; expected Never [arg-type]
Note that this sample works in the pyright playground.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the self-contained reproducer in the issue and run it against mypy 1.6.1 and 1.7.0 or later to confirm the regression. Trace generic Protocol inference involving ParamSpec, TypeVar, and Concatenate; done means the example no longer reports Never for the argument and the regression is covered.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 38/100