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lru_cache classmethod: variable has type "Self" after assignment #18993

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Redoubts opened this issue Apr 28, 2025 · 1 comment · May be fixed by #19025
Open

lru_cache classmethod: variable has type "Self" after assignment #18993

Redoubts opened this issue Apr 28, 2025 · 1 comment · May be fixed by #19025
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bug mypy got something wrong topic-self-types Types for self

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@Redoubts
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https://mypy-play.net/?mypy=latest&python=3.11&gist=81326668cbc8056d8f3b44d3396ea0d0

If I have a lru_cache decorated classmethod, I noticed that I can get an error where the return type will be literally Self rather than the inferred class. I've seen a lot of discussion about lru_cache support, but it wasn't clear to me if this would be a dupe.

To Reproduce

from typing import Self
from functools import lru_cache

class A:
    @classmethod
    @lru_cache
    def a(cls) -> Self:
        return cls()
        
        
        
x = id(A)

if x == 1:
    aa = A.a()
else:
    aa = A() # Incompatible types in assignment (expression has type "A", variable has type "Self")  [assignment]

Expected Behavior

I expected this code to have no type errors

Actual Behavior

main.py:17: error: Incompatible types in assignment (expression has type "A", variable has type "Self") [assignment]
Found 1 error in 1 file (checked 1 source file)

Your Environment

mypy 1.15, python 3.11, stock config
@Redoubts Redoubts added the bug mypy got something wrong label Apr 28, 2025
@sterliakov
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This isn't specific to lru_cache, we leak self types from any method decorators apparently.

from typing import Callable, Self

class W[T]:
    def __init__(self, fn: Callable[..., T]) -> None: ...
    def __call__(self) -> T: ...
    
class Check:
    @W
    def foo(self) -> Self:
        ...

reveal_type(Check.foo())

https://mypy-play.net/?mypy=master&python=3.12&flags=strict&gist=674dee3d07ff816c3f12ee3ab5c86f1d

@sterliakov sterliakov added the topic-self-types Types for self label Apr 29, 2025
This was referenced May 4, 2025
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Bind self-types in checkmember for decorated classmethods sterliakov/mypy
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@Redoubts @sterliakov