pandas cumulative sum column

1import pandas as pd
2from random import randint
3df = pd.DataFrame(data=[{'duration': randint(0,3)} for _ in range(5)])
4df.head()
5#   	duration
6#   0	0
7#   1	2
8#   2	1
9#   3	0
10#   4	3
11df['cum_dur'] = df.duration.cumsum()
12df.head()
13
14#       duration	cum_dur
15#   0	0	        0
16#   1	2	        2
17#   2	1	        3
18#   3	0	        3
19#   4	3	        6