python - pandas group by and sequence

Question

I have a data frame with a category column.

For each class within the category column, I would like a repeating sequence from 1 to n. For example, in the below table, for each unique value in the category column (e.g. a, b, c etc.), I would like to have a repeating sequence from 1 to 3 in the corresponding sequence column

id	category	sequence
1	a	1
2	a	2
3	a	3
4	a	1
5	a	2
6	a	3
7	b	1
8	b	2
9	b	3
10	b	1
11	b	2
12	b	3

Answer 1

Use cumcount:

df['sequence'] = (df.groupby('category').cumcount() % 3) + 1
print(df)

Output

    id category  sequence
0    1        a         1
1    2        a         2
2    3        a         3
3    4        a         1
4    5        a         2
5    6        a         3
6    7        b         1
7    8        b         2
8    9        b         3
9   10        b         1
10  11        b         2
11  12        b         3

As an alternative:

df['sequence'] = df.groupby('category').cumcount().mod(3).add(1)