Comment by notatallshaw

> Don't write functions that support hundreds of input data types

But by it's nature, duck typing supports an unbounded number of input types and is what Python was built on.

You've already decided duck typing is wrong and strict type adherence is correct, which is fine, but that doesn't fit the vast history of Python code, or in fact many of the core Python libraries.

And yet, some Python users insistent on type hinting very dynamic Python code while trying to keep how dynamic it is.

Challenge accepted.

    from dataclasses import dataclass
    from typing import Protocol, Self, TypeVar

    class Semigroup(Protocol):
        def __add__(self, other: Self) -> Self:
            ...

    T = TypeVar("T", bound=Semigroup)
    def join_stuff(first: T, *rest: T) -> T:
        accum = first
        for x in rest:
            accum += x
        return accum

    @dataclass
    class C:
        x: int

    @dataclass
    class D:
        x: int
        def __add__(self, other: Self) -> Self:
            return type(self)(self.x + other.x)

    @dataclass
    class E:
        x: int
        def __add__(self, other: Self) -> Self:
            return type(self)(self.x + other.x)

    _: type[Semigroup] = D
    _ = E

    def doit() -> None:
        print(join_stuff(1,2,3))
        print(join_stuff((1,), tuple(), (2,)))
        print(join_stuff("a", "b", "c"))
        print(join_stuff(D(1), D(2)))
        print(join_stuff(D(1), 3))
        print(D(1) + 3) # caught by mypy
        print(D(1) + E(3)) # caught by mypy
        print(join_stuff(1,2,"a")) # Not caught by mypy
        print(join_stuff(C(1), C(2))) # caught by mypy
    doit()

Now, this doesn't quite work to my satisfaction. Mypy lets you freely mix and match values of incompatible types, and I don't know how to fix that. Basically, if you directly try to add a D and an int, mypy will yell at you, but there's no way I've found to insist that the arguments to join_stuff, in addition to being Semigroups, are all of the compatible types. It looks like mypy is checking join_stuff as if Semigroup were a concrete class, so once you're inside join_stuff, the actual types of the arguments become irrelevant.

However, it will correctly tell you that it can't accept arguments that don't define addition at all, and that's better than nothing.