class needs to refer a different class

[Gribouillis]

I think there is way too much code in _zio.__init__() and most of this code is not directly related to the instance being created. Only three attributes are set in this method, namely self.realname, self.tmpname, self.allfiles. I think most of this method's code should be written in an external "factory" function named for example _new_io() and the structure could be

def _new_io(name=None,mode='r',compresslevel=None,*,
            buffering=-1,
            compress=None,
            closefd=True,
            check=-1,
            encoding=None,
            errors=None,
            format=None,
            newline=None,
            tempname=False,
    ):
    ...
    zio = _zio(realname, tmpname, allfiles)
    ...
    return zio

class _zio(io.IOBase):
    def __init__(self, realname, tmpname, allfiles):
        self.realname = realname
        self.tmpname = tmpname
        self.allfiles = allfiles
    ...

zopen() and ztopen() would invoke _new_io() instead of _zio() and the advantage is that the body of _new_io() can now incorporate special cases where it returns something different from a _zio() instance, perhaps another instance of io.IOBase().