{{ce:1,cn:[1,2]},{ce:2, cn:[2,3]}}

>>> import itertools
>>> 
>>> lst = [{'cn': 1, 'ce': 1},
...        {'cn': 2, 'ce': 1},
...        {'cn': 2, 'ce': 2},
...        {'cn': 3, 'ce': 2}]
>>> 
>>> out = []
>>> 
>>> grp = itertools.groupby(lst, lambda i: i['ce'])
>>> for i, it in grp:
...     num_lst = sorted(d['cn'] for d in it)
...     out.append({'ce': i, 'cn': num_lst})
... 
>>> print(out)
[{'ce': 1, 'cn': [1, 2]}, {'ce': 2, 'cn': [2, 3]}]
>>>