The generic type of an inner function cannot be made to match the generic type of an enclosing function

With the following example:

from typing import Callable, Generic, Type, TypeVar

XType = TypeVar('XType', bound=int)


class C(Generic[XType]):
    def f(self, x_init: XType) -> XType:
        return x_init


def combinator(c_cls: Type[C[XType]]) -> Callable[[C[XType], XType], XType]:
    old_f = c_cls.f

    def new_f(c: C[XType], x_init: XType) -> XType:
        return old_f(c, x_init)

    return new_f

MyPy 0.910 says:

a.py:15: error: Incompatible return value type (got "XType", expected "XType")
a.py:15: error: Argument 1 has incompatible type "C[XType]"; expected "C[XType]"
a.py:15: error: Argument 2 has incompatible type "XType"; expected "XType"

Ugh! This is a bad error message, and pretty confusing.

I think that the issue here is that the inner function binds those type variables separately (but maybe it shouldn't?), which is why there is a type mismatch: they are different XTypes. But it is definitely a bug in the error message!

@msullivan

@msullivan Thanks for taking a look. I ended up redesigning this, so it's not a priority for me anymore. Out of curiosity, how should I have annotated this?

However, thank you, MyPy people, for the great project. It's really useful!

@NeilGirdhar

Sorry for resuming this old issue but I just stumbled in (what I believe is) this same situation and I could not find a solution.

My code is:

from abc import ABC, abstractmethod
from typing import TypeVar, Iterable, Iterator

T = TypeVar('T')


class MyABC(ABC):

    @property
    @abstractmethod
    def items(self) -> Iterable[T]: ...

    def __iter__(self) -> Iterator[T]:
        for item in self.items:
            yield item

and, while running mypy command (version 0.910), I got:

 error: Incompatible types in "yield" (actual type "T", expected type "T")

I admit I am not completely sure it is the same issue as the one reported by @NeilGirdhar and that @msullivan suggested being due to the fact that "they are different T type", but it seemed so and I couldn't find a way to change my code to avoid this error.
In particular, I need to bind the type of the elements in 'items' to the YieldType of 'iter' , but I didn't find the correct solution to do that.

@NicolaDonelli

@NicolaDonelli Did you try inheriting from Generic[T]? (And you should probably inherit from Iterable[T] also, and replace the for loop with a simple yield from self.items.

from abc import abstractmethod
from typing import TypeVar, Iterable, Iterator, Generic
from collections.abc import Iterable

T = TypeVar('T')


class MyABC(Iterable[T], Generic[T]):
    @property
    @abstractmethod
    def items(self) -> Iterable[T]: ...

    def __iter__(self) -> Iterator[T]:
        yield from self.items

It may be worth considering filing an issue to improve the error message for this case.

@NeilGirdhar

@NeilGirdhar, nope I didn't try before and effectively your proposed solution works like a charm, thanks a lot!

For what it takes the issue about the error message, would you prefer me to open it or you'd prefer to open it yourself?

thanks a lot!

You're very welcome.

would you prefer me to open it or you'd prefer to open it yourself?

I'm not opening it.

Looks like the error message is fixed!

Navigation Menu

Repository navigation

The generic type of an inner function cannot be made to match the generic type of an enclosing function #8696

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

The generic type of an inner function cannot be made to match the generic type of an enclosing function #8696

Description

Activity

msullivan commented on Apr 29, 2020

NeilGirdhar commented on Apr 29, 2020

NicolaDonelli commented on Aug 30, 2021

NeilGirdhar commented on Aug 30, 2021

NicolaDonelli commented on Aug 30, 2021

NeilGirdhar commented on Aug 31, 2021

NeilGirdhar commented on Oct 15, 2024

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions