[red-knot] Failed to assign to a callable using `Unpack`

[dhruvmanila]

Summary

from typing import Callable, TypeVarTuple, Unpack


def some_func() -> None: ...


T = TypeVarTuple("T")


def unpack(c: Callable[[Unpack[T]], object]):
    pass


def _():
    # red-knot: Argument to this function is incorrect: Expected `(*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> object`, found `def some_func() -> None` [lint:invalid-argument-type]
    unpack(some_func)

Version

No response

[carljm]

Tagging "help wanted", would be really useful to get some exploration of why we are not considering the one callable assignable to the other one here.

[dhruvmanila]

(I'll take a quick look this morning and will reply.)

[dhruvmanila]

Hmm, actually this diagnostic seems correct, reduced version of the above: https://types.ruff.rs/b8f87ca7-7a08-4bb2-aed8-7dbb1d800242

from typing import Any
from knot_extensions import CallableTypeOf, static_assert, is_assignable_to

def empty() -> None: ...

def any_params(*args: Any, **kwargs: Any) -> object: ...

static_assert(is_assignable_to(CallableTypeOf[empty], CallableTypeOf[any_params]))  # false

Because, the any_params function can take 0 or more number of parameters but empty does not take any parameters which means we cannot assign empty to any_params.

Another way to look at it using protocols:

class SelfCallable(Protocol):  # Subtype
    def __call__(self) -> None: ...


class OtherCallable(Protocol):  # Supertype
    def __call__(self, *args: Any, **kwargs: Any) -> None: ...


def _(self_callable: SelfCallable, other_callable: OtherCallable):
    a: OtherCallable = self_callable
	# This call is correct because `a` is typed as a callable that takes any parameters
    # but the above assignment will mean that it will raise an error at runtime
    a(1, 2, 3) 

And, in the REPL:

>>> class SelfCallable(Protocol):  # Subtype
...     def __call__(self) -> None: ...
... 
... 
... class OtherCallable(Protocol):  # Supertype
...     def __call__(self, *args: Any, **kwargs: Any) -> None: ...
... 
... 
... def _(self_callable: SelfCallable, other_callable: OtherCallable):
...     a: OtherCallable = self_callable
...     # This call is correct but the above assignment will mean that it will raise an error
...     # at runtime
...     a(1, 2, 3) 
...     
>>> _(lambda: None, lambda *args, **kwargs: None)
Traceback (most recent call last):
  File "<python-input-4>", line 1, in <module>
    _(lambda: None, lambda *args, **kwargs: None)
    ~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<python-input-3>", line 13, in _
    a(1, 2, 3)
    ~^^^^^^^^^
TypeError: <lambda>() takes 0 positional arguments but 3 were given

[dhruvmanila]

I'm not sure why is Pyright and mypy not raising an error here but it does when this happens in an assignment statement:

• Pyright
• mypy

[carljm]

Ah right! When looking at this initially, I failed to consider the difference between a *args: Any, **kwargs: Any signature and a ... signature. The former is dynamic only with respect to argument types, not with respect to which parameters are accepted. The latter is dynamic with respect to both. And we seem to handle this correctly.