A列が1, B列が2 である行のC列の合計値

test.py

import pandas as pd

df1 = pd.DataFrame([[1, 1, 12],
                    [1, 2, 11],
                    [1, 1, 18],
                    [2, 2, 19],
                    [2, 1, 10],
                    [2, 2, 14],
                    [1, 1, 15],
                    [1, 2, 17],
                    [1, 1, 19]],
                   columns=['A', 'B', 'C'])

sum_of_c = df1.loc[(df1['A'] == 1) & (df1['B'] == 2), 'C'].sum()

print('sum={}'.format(sum_of_c))

出力結果

sum=28