Overloads with default arguments do not work properly

[padix-key]

I have the follwoing stub file (test.pyi):

from typing import overload

@overload
def func(
    param: None = ...
) -> int: ...

@overload
def func(
    param: int = ...
) -> str: ...

When I run mypy test.pyi, mypy shows the following error:
test.pyi:4: error: Overloaded function signatures 1 and 2 overlap with incompatible return types
The parameter param clearly has two different types. Hence, the functions should not overlap to my understanding.

mypy 0.620 has been used.

[srittau]

The problem here is that both overloads match a call of func(). Removing the default from the second overload should fix your problem:

• func() matches (only) the first overload
• func(None) matches the first overload
• func(42) matches the second overload

[padix-key]

Unfortunately, I cannot simply remove the default value from the second overload, because in my actual problem there are default parameters before this parameter, like

from typing import overload

@overload
def func(
    param1: str = "foo",
    param2: None = ...
) -> int: ...

@overload
def func(
    param1: str = "foo",
    param2: int = ...
) -> str: ...

Is there a way to fix this problem?

[ilevkivskyi]

This is not a bug, mypy shows an actual error. As @srittau mentioned, what is the type of func() (no args)? Is it int or str, because it matches both overloads. You should remove the default value (i.e. ...) from at least one of the overloads. Also please read the docs https://mypy.readthedocs.io/en/latest/more_types.html#function-overloading

[ilevkivskyi]

@padix-key you can just add more overloads that match runtime semantics of your function.

[JelleZijlstra]

Specifically, in overload 2 you can mark param2 as a required keyword-only argument by doing *, param2: str.

[henribru]

Specifically, in overload 2 you can mark param2 as a required keyword-only argument by doing *, param2: str.

@JelleZijlstra But then Mypy no longer allows you to call the function as func("foo", 3), you have to do func("foo", param2=3) even though passing them as positional arguments are fine at runtime.

[henribru]

Okay, after reading https://mail.python.org/archives/list/typing-sig@python.org/thread/3P36ZCFAMOKFJMHFULTPGRNRSGTHTOLX/ I realize this workaround actually requires three overloads to avoid the problem I mentioned:

from typing import overload

@overload
def func(
    param1: str = "foo",
    param2: None = ...
) -> int: ...

@overload
def func(
    param1: str = "foo",
    *,
    param2: int
) -> str: ...

@overload
def func(
    param1: str,
    param2: int
) -> str: ...

It feels a bit esoteric and unintuitive. Would a PR to better document the interaction between overloads and default values be welcome?

[bashtage]

It feels a bit esoteric and unintuitive. Would a PR to better document the interaction between overloads and default values be welcome?

I think a tutorial/extended help page on complex overloading would be a great addition.