Adding a grouped, aggregate nunique column to pandas dataframe

Question

I want to add an aggregate, grouped, nunique column to my pandas dataframe but not aggregate the entire dataframe. I\'m trying to do this in one line and avoid creating a ne

Answer 1

df.groupby(['track', 'type'])['id'].transform(nunique)

Implies that there is a name nunique in the name space that performs some function. transform will take a function or a string that it knows a function for. nunique is definitely one of those strings.

As pointed out by @root, often the method that pandas will utilize to perform a transformation indicated by these strings are optimized and should generally be preferred to passing your own functions. This is True even for passing numpy functions in some cases.

For example transform('sum') should be preferred over transform(sum).

Try this instead

df.groupby(['track', 'type'])['id'].transform('nunique')

demo

df = pd.DataFrame(dict(
    track=list('11112222'), type=list('AAAABBBB'), id=list('XXYZWWWW')))
print(df)

  id track type
0  X     1    A
1  X     1    A
2  Y     1    A
3  Z     1    A
4  W     2    B
5  W     2    B
6  W     2    B
7  W     2    B

df.groupby(['track', 'type'])['id'].transform('nunique')

0    3
1    3
2    3
3    3
4    1
5    1
6    1
7    1
Name: id, dtype: int64