Assign Unique ID To Each Unique Value In Group After Pandas Groupby

I have a DataFrame as following. df = pd.DataFrame({'col1': ['a','b','c','c','d','e','a','h','i','a'],'col2':['3:00','3:00','4:00','4:00','3:00','5:00','5:00','3:00','3:00','2:00']

Solution 1:

we can use pd.factorize() method:

In [170]: df['ID'] = df.groupby('col1')['col2'].transform(lambda x: pd.factorize(x)[0])

In [171]: df
Out[171]:
  col1  col2  ID
0    a  3:00   0
1    b  3:00   0
2    c  4:00   0
3    c  4:00   0
4    d  3:00   0
5    e  5:00   0
6    a  5:00   1
7    h  3:00   0
8    i  3:00   0
9    a  2:00   2