How to solve "unexpected keyword argument" with python-attrs construct, so that mypy is happy?

Question

I have a class A in a foreign library.

class A:
    def __init__(self, a: int):
        self.a = a

I would like to extend a class B with A like:

import attr

@attr.s
class B(A):
    b: int = attr.ib()

The code seems to work:

import attr

class A:
    def __init__(self, a: int):
        self.a = a

attr.s(these={
    "a": attr.ib(type=str)
}, init=True)(A)

@attr.s(kw_only=True)
class B(A):
    b: int = attr.ib()

if __name__ == "__main__":
    a = A(1)
    b = B(a=1, b=2)
    print(a)  # output: A(a=1) 
    print(b)  # output: B(a=1, b=2)

but mypy/pyright is unhappy.

> mypy file.py
error: Unexpected keyword argument "a" for "B"

Answer 1

I’m reasonably sure that the MyPy attrs plugin doesn’t support these.

Since you’re not calling super, the idiomatic way would be to just define a as an attribute on B so that attrs takes care of it.

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JFTR: if you use @attr.define or @attr.s(auto_attribs=True), you can leave out the attr.ib() call(s).