Fix explicit type for partial

[ilevkivskyi]

Fixes #17301

cc @hauntsaninja

[hauntsaninja]

from functools import partial
def foo(): ...
partial(partial, foo)()

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

kopf (https://github.com/nolar/kopf)
- kopf/_core/actions/invocation.py:127: error: Incompatible types in assignment (expression has type "partial[partial[object | Coroutine[None, None, object]]]", variable has type "partial[object | Coroutine[None, None, object | None] | None]")  [assignment]
- kopf/_core/actions/invocation.py:127: error: Argument 1 to "run" of "Context" has incompatible type "partial[object | Coroutine[None, None, object | None] | None]"; expected "Callable[[VarArg(Any), KwArg(Any)], partial[object | Coroutine[None, None, object]]]"  [arg-type]
- kopf/_core/actions/invocation.py:127: note: "partial[object | Coroutine[None, None, object | None] | None].__call__" has type "Callable[[VarArg(Any), KwArg(Any)], object | Coroutine[None, None, object | None] | None]"

ibis (https://github.com/ibis-project/ibis)
+ ibis/backends/sql/datatypes.py: error: "int" not callable  [operator]
+ ibis/backends/sql/datatypes.py:31: error: Too many arguments for "__call__"  [call-arg]
+ ibis/backends/sql/datatypes.py:48: error: Too many arguments for "__call__"  [call-arg]

[cdce8p]

This PR seems to cause a false-positive with partial and not callable.

from functools import partial

class A:
    def __init__(self, var: str, b: int, c: int) -> None: ...

partial(A, str(1))  # false-positive
reveal_type(1)

test.py: error: "str" not callable  [operator]

Note: The error doesn't include a line number which made it difficult to default. Additionally, the __init__ signature seems to need three variables for the error to be emitted.

Furthermore, it isn't limited to str(...).

from functools import partial
class B: ...

class C:
    def __init__(self, var: B, b: int, c: int) -> None: ...

partial(C, B())  # false_positive

test.py: error: "B" not callable  [operator]

[ilevkivskyi]

Yes, this is a bug, fix in #17429