Mypy incorrectly says signature is incompatible with supertype when using decorator

@ilevkivskyi

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'

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.

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

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?

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"

@stlehmann

@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.

@stlehmann

@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.

@gvanrossum

@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

We should put this somewhere in the docs.

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

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"

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

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

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]

Navigation Menu

Repository navigation

Mypy incorrectly says signature is incompatible with supertype when using decorator #3260

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Mypy incorrectly says signature is incompatible with supertype when using decorator #3260

Description

Activity

ilevkivskyi commented on Apr 26, 2017

JelleZijlstra commented on Jun 15, 2017

jdferreira commented on Jul 7, 2018

stlehmann commented on Jul 23, 2018

gvanrossum commented on Jul 23, 2018

JukkaL commented on Jul 23, 2018

stlehmann commented on Jul 24, 2018

gvanrossum commented on Jul 24, 2018

ilevkivskyi commented on Sep 26, 2018

ikelos commented on Feb 28, 2019

ilevkivskyi commented on Feb 28, 2019

ikelos commented on Feb 28, 2019

igor-petrik-invitae commented on Jan 20, 2023

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions