Provide method for the key-autocompletions in IPython.
        See http://ipython.readthedocs.io/en/stable/config/integrating.html#tab-completion
        For the details.

def _ipython_key_completions_(self) -> List[str]:
        """Provide method for the key-autocompletions in IPython.
        See http://ipython.readthedocs.io/en/stable/config/integrating.html#tab-completion
        For the details.
        """  # noqa
        item_lists = [item
                      for sublist in self._item_sources
                      for item in sublist
                      if isinstance(item, str)]
        return list(set(item_lists))