Use np.where
commission = -1
df['CASH'] = np.where(df['CASH'] != 0, df['CASH'] + commission , df['CASH'])
or df.where
i.e
df['CASH'] = df['CASH'].where(df['CASH'] == 0,df['CASH']+commission)
or df.mask
df['CASH'] = df['CASH'].mask(df['CASH'] != 0 ,df['CASH']+commission)
Date
2011-01-10 -2558.0
2011-01-11 0.0
2011-01-12 0.0
2011-01-13 -2582.0
Name: CASH, dtype: float64
%%timeit
commission = -1
df['CASH'] = np.where(df['CASH'] != 0, df['CASH'] + commission , df['CASH'])
1000 loops, best of 3: 750 µs per loop
%%timeit
df['CASH'].mask(df['CASH'] != 0 ,df['CASH']+commission)
1000 loops, best of 3: 1.45 ms per loop
%%timeit
df['CASH'].where(df['CASH'] == 0,df['CASH']+commission)
1000 loops, best of 3: 1.55 ms per loop
%%timeit
df.loc[df['CASH'] != 0, 'CASH'] += commission
100 loops, best of 3: 2.37 ms per loop