Groupby in python pandas: Fast Way

Question

I want to improve the time of a groupby in python pandas. I have this code:

df[\"Nbcontrats\"] = df.groupby([\'Client\', \'Month\'])[\'Contrat\         


        

Answer 1

With the DataFrameGroupBy.size method:

df.set_index(['Client', 'Month'], inplace=True)
df['Nbcontrats'] = df.groupby(level=(0,1)).size()
df.reset_index(inplace=True)

The most work goes into assigning the result back into a column of the source DataFrame.