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Dhruv Manilawala 0361021863 [red-knot] Understand typing.Callable (#16493 )

## Summary

Part of https://github.com/astral-sh/ruff/issues/15382

This PR implements a general callable type that wraps around a
`Signature` and it uses that new type to represent `typing.Callable`.

It also implements `Display` support for `Callable`. The format is as:
```
([<arg name>][: <arg type>][ = <default type>], ...) -> <return type>
```

The `/` and `*` separators are added at the correct boundary for
positional-only and keyword-only parameters. Now, as `typing.Callable`
only has positional-only parameters, the rendered signature would be:

```py
Callable[[int, str], None]
# (int, str, /) -> None
```

The `/` separator represents that all the arguments are positional-only.

The relationship methods that check assignability, subtype relationship,
etc. are not yet implemented and will be done so as a follow-up.

## Test Plan

Add test cases for display support for `Signature` and various mdtest
for `typing.Callable`.

2025-03-08 03:58:52 +00:00

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Callable

References:

https://typing.readthedocs.io/en/latest/spec/callables.html#callable

TODO: Use collections.abc as importing from typing is deprecated but this requires support for * imports. See: https://docs.python.org/3/library/typing.html#deprecated-aliases.

Invalid forms

The Callable special form requires exactly two arguments where the first argument is either a parameter type list, parameter specification, typing.Concatenate, or ... and the second argument is the return type. Here, we explore various invalid forms.

Empty

A bare Callable without any type arguments:

from typing import Callable

def _(c: Callable):
    reveal_type(c)  # revealed: (...) -> Unknown

Invalid parameter type argument

When it's not a list:

from typing import Callable

# error: [invalid-type-form] "The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`"
def _(c: Callable[int, str]):
    reveal_type(c)  # revealed: (...) -> Unknown

Or, when it's a literal type:

# error: [invalid-type-form] "The first argument to `Callable` must be either a list of types, ParamSpec, Concatenate, or `...`"
def _(c: Callable[42, str]):
    reveal_type(c)  # revealed: (...) -> Unknown

Or, when one of the parameter type is invalid in the list:

def _(c: Callable[[int, 42, str, False], None]):
    # revealed: (int, @Todo(number literal in type expression), str, @Todo(boolean literal in type expression), /) -> None
    reveal_type(c)

Missing return type

Using a parameter list:

from typing import Callable

# error: [invalid-type-form] "Special form `typing.Callable` expected exactly two arguments (parameter types and return type)"
def _(c: Callable[[int, str]]):
    reveal_type(c)  # revealed: (int, str, /) -> Unknown

Or, an ellipsis:

# error: [invalid-type-form] "Special form `typing.Callable` expected exactly two arguments (parameter types and return type)"
def _(c: Callable[...]):
    reveal_type(c)  # revealed: (...) -> Unknown

More than two arguments

We can't reliably infer the callable type if there are more then 2 arguments because we don't know which argument corresponds to either the parameters or the return type.

from typing import Callable

# error: [invalid-type-form] "Special form `typing.Callable` expected exactly two arguments (parameter types and return type)"
def _(c: Callable[[int], str, str]):
    reveal_type(c)  # revealed: (...) -> Unknown

Simple

A simple Callable with multiple parameters and a return type:

from typing import Callable

def _(c: Callable[[int, str], int]):
    reveal_type(c)  # revealed: (int, str, /) -> int

Nested

A nested Callable as one of the parameter types:

from typing import Callable

def _(c: Callable[[Callable[[int], str]], int]):
    reveal_type(c)  # revealed: ((int, /) -> str, /) -> int

And, as the return type:

def _(c: Callable[[int, str], Callable[[int], int]]):
    reveal_type(c)  # revealed: (int, str, /) -> (int, /) -> int

Gradual form

The Callable special form supports the use of ... in place of the list of parameter types. This is a gradual form indicating that the type is consistent with any input signature:

from typing import Callable

def gradual_form(c: Callable[..., str]):
    reveal_type(c)  # revealed: (...) -> str

Using `typing.Concatenate`

Using Concatenate as the first argument to Callable:

from typing_extensions import Callable, Concatenate

def _(c: Callable[Concatenate[int, str, ...], int]):
    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int

And, as one of the parameter types:

def _(c: Callable[[Concatenate[int, str, ...], int], int]):
    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int

Using `typing.ParamSpec`

Using a ParamSpec in a Callable annotation:

from typing_extensions import Callable

# TODO: Not an error; remove once `ParamSpec` is supported
# error: [invalid-type-form]
def _[**P1](c: Callable[P1, int]):
    reveal_type(c)  # revealed: (...) -> Unknown

And, using the legacy syntax:

from typing_extensions import ParamSpec

P2 = ParamSpec("P2")

# TODO: Not an error; remove once `ParamSpec` is supported
# error: [invalid-type-form]
def _(c: Callable[P2, int]):
    reveal_type(c)  # revealed: (...) -> Unknown

Using `typing.Unpack`

Using the unpack operator (*):

from typing_extensions import Callable, TypeVarTuple

Ts = TypeVarTuple("Ts")

def _(c: Callable[[int, *Ts], int]):
    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int

And, using the legacy syntax using Unpack:

from typing_extensions import Unpack

def _(c: Callable[[int, Unpack[Ts]], int]):
    reveal_type(c)  # revealed: (*args: @Todo(todo signature *args), **kwargs: @Todo(todo signature **kwargs)) -> int

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