Callable attribute is considered a method

[NiklasRosenstein]

Small example to illustrate the issue:

import typing as t
from dataclasses import dataclass, field

@dataclass
class Example:
  a: t.Callable[[], int] = field(default=lambda: 42)

  def call_a(self) -> None:
    self.a()   # Attribute function "a" with type "Callable[[], int]" does not accept self argument

Curiously, adding Example as the first argument to the type of a makes the call be recognized OK but the default not.

from typing import t
from dataclasses import dataclass, field

@dataclass
class Example:
  a: t.Callable[['Example'], int] = field(default=lambda: 42)  # Incompatible types in assignment (expression has type "Callable[[], int]", variable has type "Callable[[Example], int]")

  def call_a(self):
    self.a()

Your Environment

• Mypy version used: mypy 0.782
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.7
• Operating system and version: Debian WSL

[gvanrossum]

Hm, this is unfortunate. I think it's due to mypy not really understanding the difference between class and instance attributes.

I expect it'll be hard to fix. In the meantime, here's a weird workaround: if you make the type Optional and check for None before the call, it works:

class Example:
    a: Optional[Callable[[int], int]]
    def f(self) -> int:
        assert self.a is not None
        return self.a(42)

[NiklasRosenstein]

I just found another workaround using a Protocol:

if t.TYPE_CHECKING:
  class AProtocol(t.Protocol):
    def __call__(self) -> int:
      pass

@dataclass
class Example:
  a: 'AProtocol' = field(default=lambda: 42) 

I feel like there should be a way for mypy to distinguish these cases, unless there are conflicting use cases (which I am not aware of yet). But I haven't taken a look into the related mypy code.

Edit Ctrl+Enter too early again 😄 Completed the comment

[troiganto]

I'm hitting the same issue, but with a different error. In case anyone else is affect, here is the MWE:

from dataclasses import dataclass
from typing import Callable


@dataclass
class Example:
    func: Callable[[str], str]


def the_func(s: str) -> str:
    return s


example = Example(the_func)
string = example.func("passthrough")
print(string)

This runs without problems and should pass Mypy, but instead produces the following errors:

error: Invalid self argument "Example" to attribute function "func" with type "Callable[[str], str]"
error: Too many arguments

As in the original issue, one workaround is to declare the field as Optional and assert that it is not None before usage.

My understanding is that because func is declared on the class, Mypy thinks that it is a class attribute (and not an instance attribute). If this were true, example.func would indeed give a bound method. But because func exists on the instance, we get the_func back unmodified.

[hauntsaninja]

Fixed by #10548