To get statistics for each group using Python Pandas GroupBy, we can call the size method.
For instance, we write
df.groupby(['col1', 'col2']).size().reset_index(name='counts')
to call groupby with an array of columns.
Then we call size to get the row counts.
And then we call reset_index to return the values in a data a frame in the 'counts' column.
