title: Python for loop returns first value
body: <p>Background:  the code is supposed to reference a ticker symbol and the time of a trade, and then pull in the subsequent closing prices after the time of the trade (next_price_1m and next_price_2m)</p>
  
  <p>Problem: when a ticker repeats, the next_price_1m and next_price_2m repeat from the prior call for that ticker, even though the time of the trade has changed.  My initial call to get barset is working, but only for the first instance of the ticker.</p>
  
  <p>Example output:</p>
  
  <pre><code>symbol | transaction_time | next price 1m | next price 2m |  
   --------|------------------|---------------|---------------|-- 
    JPM    | 10:00 a.m.       | $90           | $91           |  
    SPY    | 10:25 a.m.       | $260          | $261          |  
    JPM    | 11:37 a.m.       | $90           | $91           |  
    AAPL   | 2:25 p.m.        | $330          | $335          |  
    JPM    | 3:02 p.m.        | $90           | $91           |  
  </code></pre>
  
  <p>JPM should have different next_price_1m and next_price_2m on 2nd and 3rd calls.</p>
  
  <p>Code:</p>
  
  <pre><code>trades_list = api.get_activities(date='2020-04-06')
  data = []
  for trade in trades_list:
    my_list_of_trade_order_ids = trade.order_id
    price = trade.price
    qty = trade.qty
    side = trade.side
    symbol = trade.symbol
    transaction_time = trade.transaction_time
    client_order_id = api.get_order(trade.order_id).client_order_id
    barset = api.get_barset(timeframe='minute',symbols=trade.symbol,limit=15,after=trade.transaction_time)
    df_bars = pd.DataFrame(barset)
    next_price_1m = df_bars.iat[0,0].c
    next_price_2m = df_bars.iat[1,0].c
    data.append({'price':price, 'qty':qty, 'side':side,'symbol':symbol,'transaction_time':transaction_time, 'client_order_id':client_order_id, 'next price 1m':next_price_1m,'next price 2m':next_price_2m})
  df = pd.DataFrame(data)
  df
  </code></pre>