我有一个数据框 df ，如下所示：
Order Type Quantity
2015-04-30 Buy 200
2015-05-06 Buy 168
2015-05-08 Sell 368
2015-06-04 Buy 126
2015-06-10 Buy 129
2015-06-17 Sell 255
2015-06-18 Buy 126
2015-06-19 Sell 126
2015-11-06 Buy 159
2016-04-01 Buy 218
2016-06-01 Buy 169
我尝试根据订单类型值更改 Quantity 列中值的符号（即保持买入为正，卖出为负）：
df.loc[df["Order Type"] == "Sell", "Quantity"] *= -1
但是，这会产生一些意想不到的结果（参见第二个和第三个卖出值）：
Order Type Quantity
2015-04-30 Buy 200
2015-05-06 Buy 168
2015-05-08 Sell -368
2015-06-04 Buy 126
2015-06-10 Buy 129
2015-06-17 Sell -368
2015-06-18 Buy 126
2015-06-19 Sell -368
2015-11-06 Buy 159
2016-04-01 Buy 218
2016-06-01 Buy 169
我可以通过添加额外的“Sign”列来解决这个问题：
df['Sign'] = 1
df.loc[df["Order Type"] == "Sell", "Sign"] = -1
df['Quantity'] *= df['Sign']
但无论如何都想弄清楚事情的原因。