df.groupby('a').count() code example

Example 1: group by count dataframe

df.groupby(['col1', 'col2']).size().reset_index(name='counts')

Example 2: group by pandas count

df[['col1', 'col2', 'col3', 'col4']].groupby(['col1', 'col2']).agg(['mean', 'count'])