apply cumulative sum on particular column pandas code example

Example: pandas cumulative sum column

import pandas as pd
from random import randint
df = pd.DataFrame(data=[{'duration': randint(0,3)} for _ in range(5)])
df.head()
#   	duration
#   0	0
#   1	2
#   2	1
#   3	0
#   4	3
df['cum_dur'] = df.duration.cumsum()
df.head()

#       duration	cum_dur
#   0	0	        0
#   1	2	        2
#   2	1	        3
#   3	0	        3
#   4	3	        6