Adding a ‘count’ column to the result of a groupby in pandas?

by

You can using size 

df.groupby(['A','B']).size()
Out[590]: 
A  B
x  p    2
y  q    1
z  r    2
dtype: int64

For your solution adding one of the columns 

df.groupby(['A','B']).B.agg('count')
Out[591]: 
A  B
x  p    2
y  q    1
z  r    2
Name: B, dtype: int64

Update : 

df.groupby(['A','B']).B.agg('count').to_frame('c').reset_index()

#df.groupby(['A','B']).size().to_frame('c').reset_index()
Out[593]: 
   A  B  c
0  x  p  2
1  y  q  1
2  z  r  2

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