code: def _fchown(self, real, fileno, uid, gid):
          
          path = self._fake_path(self._path_from_fd(fileno))
          self._chown_common(path, uid, gid)
docs: Run fake fchown code if fileno points to a sub-path of our tree.
  
          The ownership set with this fake fchown can be inspected by looking
          at the self.uid/self.gid dictionaries.