Python: Pandas Dataframe how to multiply entire column with a scalar

How do I multiply each element of a given column of my dataframe with a scalar? (I have tried looking on SO, but cannot seem to find the right solution)

Doing something like:

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

gives me a warning:

    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

Note: If possible, I do not want to be iterating over the dataframe and do something like this...as I think any standard math operation on an entire column should be possible w/o having to write a loop:

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

EDIT :

I am running 0.16.2 of Pandas

full trace:

     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

Here's the answer after a bit of research:

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

From: stackoverflow.com/q/33768122