def initialState(self, state):
        """
        Set this automaton's initial state.  Raises a ValueError if
        this automaton already has an initial state.
        """

        if self._initialState is not _NO_STATE:
            raise ValueError(
                "initial state already set to {}".format(self._initialState))

        self._initialState = state

        Set this automaton's initial state.  Raises a ValueError if
        this automaton already has an initial state.
        
stackoverflow/venv/lib/python3.6/site-packages/automat/_core.py