ruff/crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md

PEP 695 ParamSpec

ParamSpec was introduced in Python 3.12 while the support for specifying defaults was added in Python 3.13.

[environment]
python-version = "3.13"

Definition

def foo1[**P]() -> None:
    reveal_type(P)  # revealed: typing.ParamSpec

Bounds and constraints

ParamSpec, when defined using the new syntax, does not allow defining bounds or constraints.

TODO: This results in a lot of syntax errors mainly because the AST doesn't accept them in this position. The parser could do a better job in recovering from these errors.

# error: [invalid-syntax]
# error: [invalid-syntax]
# error: [invalid-syntax]
# error: [invalid-syntax]
# error: [invalid-syntax]
# error: [invalid-syntax]
def foo[**P: int]() -> None:
    # error: [invalid-syntax]
    # error: [invalid-syntax]
    pass

Default

The default value for a ParamSpec can be either a list of types, ..., or another ParamSpec.

def foo2[**P = ...]() -> None:
    reveal_type(P)  # revealed: typing.ParamSpec

def foo3[**P = [int, str]]() -> None:
    reveal_type(P)  # revealed: typing.ParamSpec

def foo4[**P, **Q = P]():
    reveal_type(P)  # revealed: typing.ParamSpec
    reveal_type(Q)  # revealed: typing.ParamSpec

Other values are invalid.

# error: [invalid-paramspec]
def foo[**P = int]() -> None:
    pass