Adding @abstractmethod __init__ to ABC breaks staticmethods

I was trying to create an ABC that also worked as a factory.

from abc import (
    ABCMeta,
    abstractmethod,
)

class WidgetWithInit(object):
    __metaclass__ = ABCMeta

    @abstractmethod
    def __init__(self, foo, bar):
        # type: (int, int) -> None
        pass

    @staticmethod
    def add(foo, bar):
        # type: (int, int) -> int
        return foo + bar

    @classmethod
    def add_class(cls, foo, bar):
        # type: (int, int) -> int
        return foo + bar

WidgetWithInit.add(1, '2')
WidgetWithInit.add_class(1, '2')

class WidgetNoInit(object):
    __metaclass__ = ABCMeta

    @staticmethod
    def add(foo, bar):
        # type: (int, int) -> int
        return foo + bar

    @classmethod
    def add_class(cls, foo, bar):
        # type: (int, int) -> int
        return foo + bar

WidgetNoInit.add(1, '2')
WidgetNoInit.add_class(1, '2')

MyPy will respond:

rt6.py:40: error: Argument 2 to "add" of "WidgetNoInit" has incompatible type "str"; expected "int"
rt6.py:41: error: Argument 2 to "add_class" of "WidgetNoInit" has incompatible type "str"; expected "int"

So adding the def __init__ seems to make the staticmethod and classmethod have a type of Any.

Fortunately I can work around it by not defining init in the ABC, and having my def create enforce the signature.

In fact mypy thinks the expression WidgetNoInit already has type Any with the definition

class WidgetWithInit(object):
    @abstractmethod
    def __init__(self, foo, bar):
        # type: (int, int) -> None
        pass

In fact __init__ having any decorator will result in the class being treated as Any, since type_object_type starts like this:

    init_method = info.get_method('__init__')
    if not init_method:
        # Must be an invalid class definition.
        return AnyType()
    else: ...

and get_method doesn't return decorated things.

Here's a possible test case

[case testAbstractInit]
from abc import abstractmethod, ABCMeta
class A(metaclass=ABCMeta):
    @abstractmethod
    def __init__(self, a: int) -> None:
        pass
class B(A):
    pass
class C(B):
    def __init__(self, a: int) -> None:
        self.c = a
a = A(1) # E: Cannot instantiate abstract class 'A' with abstract attribute '__init__'
A.c # E: "A" has no attribute "c"
b = B(2) # E: Cannot instantiate abstract class 'B' with abstract attribute '__init__'
B.c # E: "B" has no attribute "c"
c = C(3)
c.c
C.c

But I gave up on trying to fix this because the logic around recognizing decorators is too hairy.

I think this bug is deep and important. I've encountered something similar in #3227. The logic that handles decorators seems to be flawed in a way which is difficult to fix.

True, decorators are one of mypy's weak spots. We're slowly chipping away at it (e.g. #3918) but it's not easy because of special-casing in various places (both in Python and in mypy).

Support decorated constructors (#6645)

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Adding @abstractmethod init to ABC breaks staticmethods #1706

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Adding @abstractmethod __init__ to ABC breaks staticmethods #1706

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rwbarton commented on Jun 14, 2016

rwbarton commented on Jun 29, 2016

rwbarton commented on Jun 29, 2016

elazarg commented on Sep 24, 2017

gvanrossum commented on Sep 24, 2017

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Adding @abstractmethod init to ABC breaks staticmethods #1706