Rename string values automatically in pandas

Question

I have a dataframe like this:

Var Name	Val
a	11
a	1
a	2
b	3
b	4

I would like to add a column with updated and enumerated "Var Name", something like this

Var Name	Val	Var Name
a	11	a1
a	1	a2
a	2	a3
b	3	b1
b	4	b2

My idea is to enumerate the Var Name (1,2,3...) till it "recognizes" a new Var Name and start enumerating again from 1.

Answer 1

groupby + cumcount is what you need here:

df['Var Name2'] = df['Var Name'] + df.groupby('Var Name').cumcount().add(1).astype(str)

Output:

>>> df
  Var Name  Val Var Name2
0        a   11        a1
1        a    1        a2
2        a    2        a3
3        b    3        b1
4        b    4        b2

To add extra characters, such as an underscore:

df['Var Name2'] = df['Var Name'] + '_' + df.groupby('Var Name').cumcount().add(1).astype(str)