python dataframe sum by row with conditions

Question

Two dataframes, I want to group by rows, column a and b. I want to sum all the rows that has 'x' and 'z' in column b and call it 'v'.

import pandas as pd


df = pd.DataFrame({'a': ['day1','day1','day1','day2','day2','day2'], 'b': ['x','z','y','x','z','y'], 'c':[9,1,6,0,5,6], 'd':[6,9,1,3,5,9]})

The result should be:

df2 = pd.DataFrame({'a': ['day1','day1','day2','day2'], 'b': ['v','y','v','y'], 'c':[10,6,5,6], 'd':[15,1,8,9]})

Answer 1

In [106]: df.replace({'b':{'x':'v','z':'v'}}).groupby(['a','b'], as_index=False).sum()
Out[106]:
      a  b   c   d
0  day1  v  10  15
1  day1  y   6   1
2  day2  v   5   8
3  day2  y   6   9

or

In [114]: to_replace=['x','z']

In [115]: replace_with=['v','v']

In [116]: df.assign(b=df.b.replace(to_replace, replace_with)).groupby(['a','b'], as_index=False).sum()
Out[116]:
      a  b   c   d
0  day1  v  10  15
1  day1  y   6   1
2  day2  v   5   8
3  day2  y   6   9