Python：Pandas Dataframe如何将整个列与标量相乘

Question

如何将数据框给定列的每个元素与标量相乘？（我曾尝试过寻找SO，但似乎找不到正确的解决方案）

做类似的事情：

df['quantity'] *= -1 # trying to multiply each row's quantity column with -1

给我警告：

A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

注意：如果可能的话，我不想遍历数据框并执行类似的操作…因为我认为整个列上的任何标准数学运算都应该是可能的，而不必编写循环：

for idx, row in df.iterrows():
    df.loc[idx, 'quantity'] *= -1

编辑 ：

我正在跑步0.16.2熊猫

完整跟踪：

 SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self.obj[item] = s

Answer 1

经过一些研究，这是答案：

df.loc[:,'quantity'] *= -1 #seems to prevent SettingWithCopyWarning