Q:

Adding a grouped, aggregate nunique column to pandas dataframe

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 new aggregated object and merging that, etc.
my df has track, type, and id. I want the number of unique ids for each track/type combination as a new column in the table (but not collapse track/type combos in the resulting df). Same number of rows, 1 more column.
something like this isn't working:
df['n_unique_id'] = df.groupby(['track', 'type'])['id'].nunique()

nor is
df['n_unique_id'] = df.groupby(['track', 'type'])['id'].transform(nunique)

this last one works with some aggregating functions but not others. the following works (but is meaningless on my dataset):
df['n_unique_id'] = df.groupby(['track', 'type'])['id'].transform(sum)

in R this is easily done in data.table with
df[, n_unique_id := uniqueN(id), by = c('track', 'type')]

thanks!

A:

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

