python - list to dictionary conversion with multiple values per key?

Question

I have a Python list which holds pairs of key/value:

l = [[1, 'A'], [1, 'B'], [2, 'C']]

I want to convert the list into a dictionary, where multiple values per key would be aggregated into a tuple:

{1: ('A', 'B'), 2: ('C',)}

The iterative solution is trivial:

l = [[1, 'A'], [1, 'B'], [2, 'C']]
d = {}
for pair in l:
    if pair[0] in d:
        d[pair[0]] = d[pair[0]] + tuple(pair[1])
    else:
        d[pair[0]] = tuple(pair[1])

print(d)

{1: ('A', 'B'), 2: ('C',)}

Is there a more elegant, Pythonic solution for this task?

Answer 1

from collections import defaultdict

d1 = defaultdict(list)

for k, v in l:
    d1[k].append(v)

d = dict((k, tuple(v)) for k, v in d1.items())

d contains now {1: ('A', 'B'), 2: ('C',)}

d1 is a temporary defaultdict with lists as values, which will be converted to tuples in the last line. This way you are appending to lists and not recreating tuples in the main loop.