How to create a new column that increments within a subgroup of a group in Python?

I have a problem where I need to group the data by two groups, and attach a column that sort of counts the subgroup.

Example dataframe looks like this:

Expected output after attaching the new column is as follows:

colA colB colC
 1     a    1
 1     a    1
 1     c    2
 1     c    2
 1     f    3
 1     z    4
 1     z    4
 1     z    4
 2     a    1
 2     b    2
 2     b    2
 2     b    2
 3     c    1  
 3     d    2 
 3     k    3    
 3     k    3 
 3     m    4
 3     m    4
 3     m    4

I tried the following but I cannot get this trivial looking problem solved:

Solution 1 I tried that does not give what I am looking for:

df['ONES']=1
df['colC']=df.groupby(['colA','colB'])['ONES'].cumcount()+1
df.drop(columns='ONES', inplace=True)

I also played with transform, and cumsum functions, and apply, but I cannot seem to solve this. Any help is appreciated.

Edit: minor error on dataframes.

Edit 2: For simplicity purposes, I showed similar values for column B, but the problem is within a larger group (indicated by colA), colB may be different and therefore, it needs to be grouped by both at the same time.

Edit 3: Updated dataframes to emphasize what I meant by my second edit. Hope this makes it more clear and reproduceable.

Solution

You could use groupby + ngroup:

df['colC'] = df.groupby('colA').apply(lambda x: x.groupby('colB').ngroup()+1).droplevel(0)

Output:

    colA colB  colC
0      1    a     1
1      1    a     1
2      1    c     2
3      1    c     2
4      1    f     3
5      1    z     4
6      1    z     4
7      1    z     4
8      2    a     1
9      2    b     2
10     2    b     2
11     2    b     2
12     3    c     1
13     3    d     2
14     3    k     3
15     3    k     3
16     3    m     4
17     3    m     4
18     3    m     4