s1 = pd.Series([1,2,3,4,5,6], index=pd.date_range('20130102', periods=6))
df['F'] = s1              # Setting a new column automatically aligns the data by the indexes 
df.at[dates[0],'A'] = 0   # Setting values by label
df.iat[0,1] = 0           # Setting values by position
df.loc[:,'D'] = np.array([5] * len(df))   # Setting by assigning with a numpy array
df2 = df.copy()
df2[df2 > 0] = -df2       # A `where` operation with setting.