count unique of records in a dataframe

Question

I have a dataframe with this form:

      CLIENTCODE  YEARMONTH
0           1     201301
1           1     201301
2           2     201301
3           1     201302
4           2     201302
5           2     201302
6           3     201302

I would like to add a new columns 'count' which contain the unique number of 'YEARMONTH' for each 'CLIENTCODE'.

So I did like this in python :

table.groupby('YEARMONTH').CLIENTCODE.nunique()

But I don't know how to add this as a columns in a dataframe.

Can you help me please?

Thanks