I'm looking for a fast pandas way of labeling sections within a dataframe.

Suppose I have a dataframe column A with some strings in it, I'd like to create a new column B that tags the sections incrementally between the keyword 'hi' like so:

A   B
hi  
a   1
b   1
hi
d   2
f   2
g   2
hi

CodePudding user response：

df.assign(C = df['A'].eq('hi').cumsum().mask(df['B'].isna()))
Out: 
    A    B    C
0  hi  NaN  NaN
1   a  1.0  1.0
2   b  1.0  1.0
3  hi  NaN  NaN
4   d  2.0  2.0
5   f  2.0  2.0
6   g  2.0  2.0
7  hi  NaN  NaN