How to obtain unique count of categorical variable based on another categorical variable?

Question

I have a dataframe with date and cat_1 variables among other variables. I want to get number of unique values of cat_1, per date. Here is an example: Below is a toy dataframe.

         date foo
0  2019-04-21   a
1  2019-04-21   b
2  2019-04-23   c
3  2019-05-01   c
4  2019-05-01   c
5  2019-04-23   a

I want to get is the following:

2019-04-21    2
2019-04-23    2
2019-05-01    1

I got it, but I think it is a bit convoluted. My code:

a.groupby(by='date')['foo'].apply(np.unique).apply(len)

Is there any other straightforward way to get the same answer?

Thanks

Answer 1

Try nunique(). That should do it. Here is a toy example:

import pandas as pd

df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
                          'foo', 'bar', 'foo', 'foo'],
                   'B' : ['one', 'one', 'two', 'three',
                          'two', 'two', 'one', 'two']})

print(df)

gives

     A      B
0  foo    one
1  bar    one
2  foo    two
3  bar  three
4  foo    two
5  bar    two
6  foo    one
7  foo    two

and

print(df.groupby('A')['B'].nunique())

then gives

A
bar    3
foo    2
Name: B, dtype: int64

For more details check this question on SO.