import pandas as pd
df = pd.DataFrame({'a': [1,2,3], 'b': [2,3,4], 'c':['dd','ee','ff'], 'd':[5,9,1]})
df['e'] = df[['a', 'b', 'd']].sum(axis = 1)
#########################
## RESULT ##
#########################
# a b c d e # COLUMNS
# 0 1 2 dd 5 8 # ROW
# 1 2 3 ee 9 14 # ROW
# 2 3 4 ff 1 8 # ROW
#########################