How to count distinct values in a column of a pandas group by object?

Question

I have a pandas data frame and group it by two columns (for example col1 and col2). For fixed values of col1 and col2 (i.e. for a group) I can have several different values in the col3. I would like to count the number of distinct values from the third columns.

For example, If I have this as my input:

1  1  1 1  1  1 1  1  2 1  2  3 1  2  3 1  2  3 2  1  1 2  1  2 2  1  3 2  2  3 2  2  3 2  2  3 

I would like to have this table (data frame) as the output:

1  1  2 1  2  1 2  1  3 2  2  1 

Answer 1

df.groupby(['col1','col2'])['col3'].nunique().reset_index() 

Answer 2

In [17]: df Out[17]:      0  1  2 0   1  1  1 1   1  1  1 2   1  1  2 3   1  2  3 4   1  2  3 5   1  2  3 6   2  1  1 7   2  1  2 8   2  1  3 9   2  2  3 10  2  2  3 11  2  2  3  In [19]: df.groupby([0,1])[2].apply(lambda x: len(x.unique())) Out[19]:  0  1 1  1    2    2    1 2  1    3    2    1 dtype: int64