Mypy: "multiple values for keyword argument" with star expansion

[Eugeny]

MyPy seems to be unaware of the length of a Tuple when wrapped in a Union:

from typing import Union, Tuple


def get_tuple() -> Union[Tuple[int, int], Tuple[None, None]]:
    return (1, 2)


def process_tuple(one, two, flag=False):
    print(one, two, flag)


process_tuple(*get_tuple(), flag=True)

1.py:12: error: "process_tuple" gets multiple values for keyword argument "flag"

Same code without a unionized type (def get_tuple() -> Tuple[int, int]:) works.

[ilevkivskyi]

Oh, it looks like one more place where we need to special-case union types.

[ilevkivskyi]

I think this is a pretty rare situation, so I set priority to low.

[JukkaL]

Updated priority to normal since this is a clear bug and we'd be happy to accept a PR that fixes this (at least if the fix is clean enough).

[t-mart]

To add to the severity, consider this ubiquitous use case of initializing a constructor with custom keyword arguments:

from typing import Any


class Foo:
    def __init__(self, a: str, b: str = 'Foo kwarg', c: str = 'Foo kwarg') -> None:
        print(f"{a=} {b=} {c=}")


class Bar(Foo):
    def __init__(self, *args: Any, **kwargs: Any) -> None:
        super().__init__(*args, b='Bar kwarg', **kwargs)


Bar('my arg')
# prints:
#    a='my arg' b='Bar kwarg' c='Foo kwarg'

The above code is syntactically correct and runs fine, but mypy produces this on the super() line:

c:\Users\tim\code\blog-py\generator\foo.py:11: error: "__init__" of "Foo" gets multiple values for keyword argument "b"

For now, I will unfortunately # type: ignore this line in my code.

[rmorshea]

@t-mart technically I think it would be more correct to have Bar's init be:

def __init__(self, b='Bar kwarg', *args: Any, **kwargs: Any) -> None:
    super().__init__(*args, b=b, **kwargs)

Since in your version above it would be possible to have multiple arguments for b passed to Foo. Unfortunately though, this re-writing still produces the same error.

[alexey-tereshenkov-oxb]

As an additional example, quite common when some of the parameters are loaded from some external source and then you need to supply the rest of the parameters explicitly:

class MyClass:
    def __init__(self, param1: str, param2: str, param3: str):
        self.param1 = param1
        self.param2 = param2

data = {"param1": "value1", "param2": "value2"}  # coming from some external source
result = MyClass(**data, param3="value3")

would give

error: "MyClass" gets multiple values for keyword argument "param3"

[hauntsaninja]

@alexey-tereshenkov-oxb your example passes on mypy master :-)

[alexey-tereshenkov-oxb]

@alexey-tereshenkov-oxb your example passes on mypy master :-)

Thanks for pinging :) I am on mypy 0.790