cdef class C:

    cdef int i

    def foo(self):

        self.i = 42

    def get_i(self):

        return self.i

def cast_cast_cast(arg):

    """

    >>> x = C()

    >>> x.foo()

    >>> cast_cast_cast(x) == x

    True

    >>> x.get_i()

    42

    """

    cdef object x

    cdef void *p = <void *>arg

    cdef int i

    x = <object>p

    p = <void *>x

    x = (<object>p).foo

    i = (<C>p).i

    (<C>p).i = i

    return <object>p