Initilize signal history

def _add_signal_history(self, df, symbol):
        """ Initilize signal history """
        if symbol not in self.signals.keys() or len(self.signals[symbol]) == 0:
            self.signals[symbol] = [nan] * len(df.index)
        else:
            self.signals[symbol].append(nan)

        self.signals[symbol] = self.signals[symbol][-len(df.index):]
        signal_count = len(self.signals[symbol])
        df.loc[-signal_count:, 'signal'] = self.signals[symbol][-signal_count:]

        return df