Show ID that append less that 16 times

I have dataframe that shows how many times ID append during every hour of day (from 0 to 23 hour).

Let’s say it looks like this:

        ID  hour
0   1   0
1   1   0
2   1   0
3   1   0
4   1   0
...     ...     ...
10115   456     23
10116   456     23
10117   456     23
10118   456     23
10119   456     23

10120 rows × 2 columns

I want to find IDs that append less than 12 times during a day (so append less than 12 hours during a day)

I have tried:

df2.groupby("ID").hour.unique()

and output:

ID
1                                               [15, 20]
2     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
3     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
4     [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,...
5      [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]
                              ...                        
452                                                 [15]
453                                                 [15]
454                                                 [16]
455                                                 [16]
456                                                 [16]

Name: hour, Length: 2853, dtype: object

If I am thinking correctly, it shows me how many times (and in which hour) ID appear as a list.
How can I referer to all this lists that lenght is smaller than 12?

>Solution :

Use nunique instead of unique:

>>> df2.groupby("ID")['hour'].nunique().loc[lambda x: x < 12].index
Int64Index([1, 452, 453, 454, 455, 456], dtype='int64', name='ID')