code: def get_historic_trades(self, start, end, granularity, product_id='BTC-USD'):
      
      params = {
        'start':self._format_iso_time(start),
        'end':self._format_iso_time(end),
        'granularity':granularity
      }
      return self._get('products', product_id, 'candles', params=params)
docs: `<https://docs.exchange.coinbase.com/#get-historic-rates>`_
  
      :param start: either datetime.datetime or str in ISO 8601
      :param end: either datetime.datetime or str in ISO 8601
      :pram int granularity: desired timeslice in seconds
      :returns: desired data