def preauthChild(self, path):
        """
        Use me if C{path} might have slashes in it, but you know they're safe.

        @param path: A relative path (ie, a path not starting with C{"/"})
            which will be interpreted as a child or descendant of this path.
        @type path: L{bytes} or L{unicode}

        @return: The child path.
        @rtype: L{FilePath} with a mode equal to the type of C{path}.
        """
        ourPath = self._getPathAsSameTypeAs(path)

        newpath = abspath(joinpath(ourPath, normpath(path)))
        if not newpath.startswith(ourPath):
            raise InsecurePath("%s is not a child of %s" %
                               (newpath, ourPath))
        return self.clonePath(newpath)

        Use me if C{path} might have slashes in it, but you know they're safe.

        @param path: A relative path (ie, a path not starting with C{"/"})
            which will be interpreted as a child or descendant of this path.
        @type path: L{bytes} or L{unicode}

        @return: The child path.
        @rtype: L{FilePath} with a mode equal to the type of C{path}.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/python/filepath.py