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main.py

import typing as t

class C:
    pass

_C = t.TypeVar('_C', bound=C)

class A(t.Generic[_C]):
    def __init__(self, c: type[_C] | None = None) -> None:
        self._c = c or C

    @property
    def c(self) -> type[_C]:
        return self._c

    @c.setter
    def c(self, c: type[_C]) -> None:
        self._c = c