The workaround is to override _init(), which I agree is not desirable.

This is the relevant code in PurePath, which is the super class of PurePosixPath:

    @classmethod
    def _from_parsed_parts(cls, drv, root, parts, init=True):
        self = object.__new__(cls)
        self._drv = drv
        self._root = root
        self._parts = parts
        if init:
            self._init()
        return self

...

    def _init(self):
        # Overridden in concrete Path
        pass

To me, the clean way to get the desired behavior seems like it would be to have _init() call self.__init__().

    def _init(self):
        # Overridden in concrete Path
        self.__init__()

This fixes p.parent, but GithubPath() ends up calling GithubPath.__init__() twice – the first time by PurePath.__new__() calling PurePath._init() and the second time by the GithubPath object creation.