[PEP 696] default substitution not applied correctly when parametrizing a generic type.

[randolf-scholz]

Bug report

Bug description:

PEP 696 allows the default of a type-parameter to depend on other type parameters, however, this substitution is not applied at runtime:

class Bar[T, S=T]: ...

print(Bar[int])  # __main__.Bar[int, T]

The result should be __main__.Bar[int, int]¹

This is likely the root cause for #140596. Tested with 3.13.8, 3.14.0 and 3.15.0a1+

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Footnotes

1. pyright playground, mypy-playground, ty-playground, pyrefly sandbox ↩

[Viicos]

This might be tricky to support, e.g. this example:

T = TypeVar("T")
ListDefaultT = TypeVar("ListDefaultT", default=list[T])

class Bar(Generic[T, ListDefaultT]):
    def __init__(self, x: T, y: ListDefaultT): ...

reveal_type(Bar)                         # type is type[Bar[T, ListDefaultT = list[T]]]
reveal_type(Bar[int])                    # type is type[Bar[int, list[int]]]

Would require replacing each (nested) type variable in the default.