New columns with incremental numbers that initial based on a diffrent column value (pandas)

Question

I want to add a column with incremental numbers for rows with the same value in a defined row;

e.g. if I would have this df

df=pd.DataFrame([['a','b'],['a','c'],['c','b']])

and I want incremental numbers for the first column. It should look like this

df=pd.DataFrame([['a','b',1],['a','c',2],['c','b',1]])

I found sql solutions but I'm working with ipython/pandas. Can someone help me?

Answer 1

Use cumcount, for name of new column use length of original columns:

print (len(df.columns))
2

df[len(df.columns)] = df.groupby(0).cumcount() + 1
print (df)
   0  1  2
0  a  b  1
1  a  c  2
2  c  b  1