everything is fine.. until it's not!

fine.py

import ctypes
import dataclasses
import faulthandler
import typing


@dataclasses.dataclass(slots=True, repr=False)
class Human:
    name: str
    age: int = 0


def main() -> None:
    faulthandler.enable()

    john = Human("John", age=33)

    john_c = typing.cast(
        "ctypes.py_object[Human]", ctypes.cast(id(john), ctypes.py_object)
    )
    ctypes.resize(john_c, 16)

    john = typing.cast(
        "ctypes.py_object[ctypes.py_object[Human]]",
        ctypes.cast(id(john_c), ctypes.py_object),
    ).value.value

    print(john)  # it's fine


if __name__ == "__main__":
    main()

segfault.py

import ctypes
import dataclasses
import faulthandler
import typing


@dataclasses.dataclass(slots=True, repr=False)
class Human:
    name: str
    age: int = 0


def main() -> None:
    faulthandler.enable()

    john = Human("John", age=33)

    john_c = typing.cast(
        "ctypes.py_object[Human]", ctypes.cast(id(john), ctypes.py_object)
    )
    ctypes.resize(john_c, 16)

    john = typing.cast(
        "ctypes.py_object[ctypes.py_object[Human]]",
        ctypes.cast(id(john_c), ctypes.py_object),
    ).value

    print(john.value)  # segfaults!


if __name__ == "__main__":
    main()