Access to instance attribute with callable type causes error

test_dc.py:

from dataclasses import dataclass, fields


@dataclass
class Foo:
    value: str


fields(Foo)[0].default_factory

$ mypy test_dc.py 
test_dc.py:9: error: Attribute function "default_factory" with type "Callable[[], Any]" does not accept self argument

python version: 3.7.1
mypy version: 0.710+dev.95a04913df54d5b1ff78fcb9e7467b798491ec4b

It is not something specific to dataclasses and in fact may be tightly related to #708.

Here is a minimal repro:

from typing import Callable

class C:
    x: Callable[[], None]

x: C
x.x

Ran into this independently. Confirmed the repro above still fails in mypy-0.740.

I notice that this works:

class Thing(NamedTuple):
    f: Callable[[], int]

Thing(lambda: 123).f()

...and this doesn't:

@dataclass
class Thing:
    f: Callable[[], int]

Thing(lambda: 123).f()

# error: Attribute function "f" with type "Callable[[], int]" does not accept self argument  [misc]

also stumbled across this one when checking whether a dataclasses.default_factory was an instance of dataclasses' _MISSING_TYPE value

Hello I have run into this when using enum.Enum:

import enum
class TestEnum2(enum.Enum):
	A = 1
	B = 2

for obj in TestEnum2:
	print(obj)

Result:

test.py:9: error: Attribute function "__iter__" with type "Callable[[], _EnumMetaType]" does not accept self argument
test.py:9: error: Attribute function "__next__" with type "Callable[[], TestEnum2]" does not accept self argument

Tested on:
mypy 0.770+dev.c90b405934cbf2142b8a575ddd89dbedf762106c (current master)
and also mypy 0.770 release

@pbabics

@pbabics I can't reproduce that error. Your line numbers don't match your code sample, so maybe you were typechecking a different file.

Yes, you are right, I was testing this with our internal plugin that shadows standard Enum type checking, sorry for confusion

@JelleZijlstra

@JelleZijlstra the error still happens in 0.780 and latest if you try on https://mypy-play.net/?mypy=latest&python=3.8


class C:
    x: Callable[[], None]

x: C
x.x

Can confirm this is still happening with a dataclass on 0.782. setattr and getattr calls do not raise the error, so I'm using them as a workaround.

Well, if you don't want to wait for a fix in mypy, here's an extremely shitty workaround:

from typing import Callable, cast

class C:
    _x: Callable[[], None]

    def x(self) -> None:
        return cast(Callable[[], None], getattr(self, "_x"))()

getattr(self, "_x") prevents a "redundant cast to Callable[[], None]" warning, and the cast is required to avoid passing the self argument to self._x.

This type-checks, but it's far from pretty.

@9999years

Is there a way to work around this for dataclasses? I have the following minimal example:

import dataclasses


@dataclasses.dataclass
class A:
    x: float = dataclasses.field(default_factory=float)

fields = dataclasses.fields(A)
assert fields[0].default_factory is not dataclasses.MISSING

Running MyPy 0.812 with all default settings gives

test.py:9: error: Attribute function "default_factory" with type "Callable[[], _T]" does not accept self argument  [misc]
Found 1 error in 1 file (checked 1 source file)

I would really rather not add # type: ignore[misc] everywhere we do something like this, and @9999years' fix won't work for us because we're using desert in conjunction with it and that would change the setting's name.

The issue with default_factory was fixed by a typeshed change (#10750 , python/typeshed#5718). The underlying logic still needs fixing though, i.e., the min repro here still reproes #6910 (comment)

If you're running into this, a callback protocol is a good workaround (see python/typeshed#5718 for an example).

@hauntsaninja

As @hauntsaninja mentioned original example was already fixed, the other example is also working now. Probably was fixed by the same PR that fixed #708

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Access to instance attribute with callable type causes error #6910

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Access to instance attribute with callable type causes error #6910

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ilevkivskyi commented on Jun 1, 2019

ilevkivskyi commented on Jun 1, 2019

Strilanc commented on Nov 27, 2019

frou commented on May 10, 2020

sinback commented on May 11, 2020

pbabics commented on May 18, 2020

JelleZijlstra commented on May 18, 2020

pbabics commented on May 18, 2020

staticdev commented on Jun 30, 2020

snejus commented on Jul 6, 2020

9999years commented on Nov 12, 2020

dargueta commented on Mar 12, 2021

hauntsaninja commented on Mar 28, 2022

ilevkivskyi commented on Aug 23, 2022

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