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?