Mypy incorrectly says signature is incompatible with supertype when using decorator

[fniessink]

When checking the code below with mypy v0.501, mypy complains that

deco.py:10: error: Signature of "a" incompatible with supertype "A"

According to @ilevkivskyi this may be related to #1441.

import functools

class A:
  @functools.lru_cache()
  def a(self) -> str: 
    return 'a'

class B(A):
  @functools.lru_cache()
  def a(self) -> str:
    return super().a() + 'b'

[ilevkivskyi]

I played with this a bit and it looks like this is specific to functools.lru_cache(), most probably since it is a class that defines a __call__ method with decorator-like signature. So this is different from #1441.

[JelleZijlstra]

I just ran into this too with a method decorated with @contextlib.asynccontextmanager (https://docs.python.org/3.7/library/contextlib.html#contextlib.asynccontextmanager).

[jdferreira]

I also ran with this issue today, precisely with the @lry_cache decorator on a subclass's method overriding a base method. Are there any solutions or workarounds at this point?

[stlehmann]

I' ve als got this issue with a stub-file for PyQt5:

class Qt:
    class Alignment: ...

class QWidget: ...

class QLayoutItem: ...

class QObject: ...

class QLayout(QObject, QLayoutItem):
    def addWidget(self, w: QWidget) -> None: ...

class QGridLayout(QLayout):
    def addWidget(
        self, widget: QWidget, row: int, column: int, alignment: Qt.Alignment = ...
    ) -> None: ...

Results in the following mypy error:

test.pyi:14: error: Signature of "addWidget" incompatible with supertype "QLayout"

[gvanrossum]

@stlehmann Mypy is right in your case, it fails Liskov. The subclass method requires more arguments than the superclass method, so it is not a proper subtype.

[JukkaL]

@stlehmann You can silence the mypy error by adding a # type: ignore comment on line 14. It won't cause mypy to ignore the signature, but it will stop complaints about the Liskov violation.

[stlehmann]

@gvanrossum Thanks for the explanation. I googled Liskov and it makes perfectly sense. However in this case I can't change the API so I will use #type: ignore to silence the error message as proposed by @JukkaL

[gvanrossum]

We should put this somewhere in the docs.

[ilevkivskyi]

I tried both examples (lru_cache() and asynccontextmanager) they both work correctly on master.

[ikelos]

I'm not sure the lru_cache() issue is resolved as of 0.670? The example provided applied lru_cache() to both methods, but this error is still raised if lru_cache() is only used on one method. Is that expected? If not, I'm happy to open a new bug for it if that's the proper way to handle it.

I found this when I had defined an abstractmethod (which therefore didn't need the lru_cache decorator).

class A: 
  def a(self) -> str: 
    return 'a'
  
class B(A): 
  @functools.lru_cache() 
  def a(self) -> str: 
    return super().a() + 'b'

results in:

mypy-test.py:9: error: Signature of "a" incompatible with supertype "A"

[ilevkivskyi]

Yes, this is a bug, but we already have an issue for this, see #5836

[ikelos]

Ok, cool. I'll follow along there, thanks! 5:)

[igor-petrik-invitae]

I am still encountering this issue with the following use case on MyPy version 0.991:

from functools import cache


class Base:
    @classmethod
    @cache
    def value(cls) -> str:
        # Some expensive default implementation
        return "[computed default]"


class Simple(Base):
    @classmethod
    def value(cls) -> str:
        return "Simple value that doesn't need caching"

error: Signature of "value" incompatible with supertype "Base" [override]