Cumulative count between two specific events in Pandas

Question

I want to count all steps of each user after 'Start' event in Pandas.

My dataset:

ID	Event
1	Start
1	Event1
1	Start
1	Event1
1	Event2
1	Start
2	Start
2	Event1
2	Start

I want:

ID	Event	Count
1	Start	1
1	Event1	2
1	Start	1
1	Event1	2
1	Event2	3
1	Start	1
2	Start	1
2	Event1	2
2	Start	1

Answer 1

Use groupby+cumsum and then cumcount:

df['Count'] = df.groupby(df['Event'].eq('Start').cumsum()).cumcount().add(1)

Output:

>>> df
   ID   Event  Count
0   1   Start      1
1   1  Event1      2
2   1   Start      1
3   1  Event1      2
4   1  Event2      3
5   1   Start      1
6   2   Start      1
7   2  Event1      2
8   2   Start      1