get list of dictionaries by combining values of on key based on value of another key

Question

I have a list [{"a":11, "b":2}, {"a":12, "b":2}, {"a":13, "b":3}, {"a":14, "b":4}]

I want to combine values of a based on values of b

I want output as [{'a':[11, 12], 'b':2}, {'a':[13], 'b':3}, {'a':[14], 'b':4}]

I have tried [{ k:list(set([d[k] for d in a])) for k in a[0] } for i in a]

Answer 1

You can create an intermediary dict to map values of b to a sub-list of the values of a , and then use a list comprehension that outputs the items of the intermediary dict as a dict with keys as 'b' and values as 'a':

mapping = {}
for d in lst:
    mapping.setdefault(d['b'], []).append(d['a'])
[{'a': v, 'b': k} for k, v in mapping.items()]

This returns:

[{'a': [11, 12], 'b': 2}, {'a': [13], 'b': 3}, {'a': [14], 'b': 4}]