decorator as class member raises "self-argument missing"

[azrdev]

I have a legitimate case for member functions not requiring a self parameter, which fails with error: Self argument missing for a non-static method (or an invalid type for self) (mypy version 0.730, python 3.7.4). MWE:

import functools
from typing import Callable

class API:
    def _login_required(meth: Callable) -> Callable:
        @functools.wraps(meth)
        def check_login(self, *args, **kwargs):
            if not self.check_login():
                self.login()
            return meth(*args, **kwargs)
        return check_login

    @_login_required
    def version(self):
        pass

It is a member function of a class, to be used as decorator.

[ilevkivskyi]

It is really interesting that this is closely related to #7191 as well. I think the solution is the same, move consistency check from definition to use site, i.e. to bind_self(). In this case we would give an error when one tries to write API()._login_required.

It looks like we have a large bunch of issues that would be fixed my moving the check to use site (plus some changes for __init__ and overloaded methods, because those a special w.r.t. self-types).

Probably we should open another meta-issue like #7724, @JukkaL what do you think?

(Btw, the new meta issue would partially depend on the previous one, because we would need more "principled" Liskov checks, like in this case _login_required would be completely excluded from Liskov check.)

[ilevkivskyi]

Probably we should open another meta-issue like #7724

Well, no we have a "meta-PR" #7860 (turned out to be smaller than expected).

[bardiharborow]

@ilevkivskyi this still seems to occur in MyPy 0.782, Python 3.8.2.

from typing import Any, Callable

class Foo:
    def decorator(func: Callable[[Any, int, int], int]):
        def modified_func(self, a: int, b: int) -> int:
            return func(self, a, b) + 1

        return modified_func

    @decorator
    def add(self, a: int, b: int) -> int:
        return a + b

if __name__ == '__main__':
    foo = Foo()
    print(foo.add(1, 2))

$ python3 foo.py
4
$ mypy .
foo.py:4: error: Self argument missing for a non-static method (or an invalid type for self)
Found 1 error in 1 file (checked 1 source file)

[Fokko]

We have a similar situation in Apache Spark: https://github.com/apache/spark/blob/master/python/pyspark/mllib/linalg/__init__.py#L472-L490

python/pyspark/mllib/linalg/__init__.py:472: error: Self argument missing for a non-static method (or an invalid type for self)
Found 1 error in 1 file (checked 186 source files)

mypy --version
mypy 0.782

[azrdev]

Should this issue be reopened?

[gvanrossum]

Probably best to open a new issue with a simple, complete repro.

[kernelmethod]

Just re-encountered this problem in mypy 0.902 with Python 3.8.

Probably best to open a new issue with a simple, complete repro.

Did anybody ever open this issue?

[mr-ubik]

@kernelmethod have you found out if the issue was opened?

[datadominik]

I'm still getting the same error, is there any update regarding class member decorators without self?

[tanlin2013]

Still have this error in July, 2022.

[nilreml]

Still have this error in February, 2023.

[ilius]

Same here:

class T_Collator(typing.Protocol):
	@classmethod
	def createInstance(loc: "T_Locale | None" = None) -> "T_Collator":
		pass

error: Self argument missing for a non-static method (or an invalid type for self) [misc]

$ mypy --version
mypy 1.4.1 (compiled: yes)

[ikonst]

@ilevkivskyi #7860 added the testBadClassLevelDecoratorHack test but presumably it works due to the type-var, not due to the check going away? (this seems to be why this issue is still a problem for many folks)

[ilevkivskyi]

The check is not going anywhere, because it is correct and, judging from the last example, useful. That PR allowed a hacky way to avoid the error while staying type-safe: that PR allows arbitrary annotation method for first argument (even completely unrelated to current class) as soon as it is a protocol. This was initially intended for typing mixins (see docs), but it apparently also helps with class level decorators, one just needs to replace Callable annotation for first argument with an equivalent callback protocol.

[ikonst]

For some reason I assumed we moved the check to call-sites.

[kxue-godaddy]

The check is not going anywhere, because it is correct and, judging from the last example, useful. That PR allowed a hacky way to avoid the error while staying type-safe: that PR allows arbitrary annotation method for first argument (even completely unrelated to current class) as soon as it is a protocol. This was initially intended for typing mixins (see docs), but it apparently also helps with class level decorators, one just needs to replace Callable annotation for first argument with an equivalent callback protocol.

Thanks for the summary! I had this typing question during work today and found the solution here. Below is a working example that I find interesting, and would like to post it here in case it's helpful to anyone:

from __future__ import annotations
from functools import wraps
from typing import Callable, Protocol


class Greeter(Protocol):
    # def __call__(_, self: A, message: str) -> None:  # this works
    def __call__(self, a: A, msg: str, /) -> None:  # also works
        ...


class A:
    def _decorator(f: Greeter) -> Callable[[A, str], None]:
        @wraps(f)
        def _f_(self: A, msg: str) -> None:
            print("-" * 40)
            f(self, msg)
            print("-" * 40)

        return _f_

    def __init__(self, name: str) -> None:
        self.name = name

    @_decorator
    def greet(self, message: str) -> None:
        print(f"{message} from {self.name}.")


a = A("random guy")
a.greet("Hello World")

I find that if Greeter.__call__ is not marked to have positional only parameters, its 2nd and 3rd arguments must be of the same names as the 1st and 2nd of A.greet (i.e. self and messge). Otherwise mypy reports an arg-type error on the line of @_decorator. I don't have a CS degree, but from what I learned by reading PEP 570, I guess this is due to the Liskov Substitution Principle. Kudos to the mypy developers for such details!