def extract_q_df(self, state_key, action_key):
        '''
        Extract Q-Value from `self.q_df`.

        Args:
            state_key:      The key of state.
            action_key:     The key of action.

        Returns:
            Q-Value.

        '''
        q = 0.0
        if self.q_df is None:
            self.save_q_df(state_key, action_key, q)
            return q

        q_df = self.q_df[self.q_df.state_key == state_key]
        q_df = q_df[q_df.action_key == action_key]

        if q_df.shape[0]:
            q = float(q_df["q_value"])
        else:
            self.save_q_df(state_key, action_key, q)
        return q
Extract Q-Value from `self.q_df`.

        Args:
            state_key:      The key of state.
            action_key:     The key of action.

        Returns:
            Q-Value.