Access to instance attribute with callable type causes error

[unknownlighter]

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

[ilevkivskyi]

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

[ilevkivskyi]

Here is a minimal repro:

from typing import Callable

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

x: C
x.x

[Strilanc]

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

[frou]

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]

[sinback]

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

[pbabics]

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

[JelleZijlstra]

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

[pbabics]

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

[staticdev]

@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

[snejus]

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.

[9999years]

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.

[dargueta]

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.

[hauntsaninja]

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).

[ilevkivskyi]

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