R's seq_len equivalent in Python [duplicate]

Question

I am trying to do an indexing within groups in my data.frame

For example, in this data frame

Col1
A
A
A
B
B
C
D
D
D

I would like to the output as the following

Col1    idx
A   1
A   2
A   3
B   1
B   2
C   1
D   1
D   2
D   3

In R, I could simply do the following using data.table df[, idx:=seq_len(.N), by=Col1]. I am having trouble finding the equivalent in Python. So far, I know I can use linspace or arrange function in the numpy's package, but I am not quite sure how to do it by groups.

Thank you in advance.

Answer 1

Use cumcount

In [289]: df['idx'] = df.groupby('Col1').cumcount().add(1)

In [290]: df
Out[290]:
  Col1  idx
0    A    1
1    A    2
2    A    3
3    B    1
4    B    2
5    C    1
6    D    1
7    D    2
8    D    3