How to groupby list of dictionary in python?

Given the list

l = [('a', 1), ('b', 2), ('a', 1), ('a', 2), ('c', 5), ('b', 3)]

how do I get the dictionary

{'a': [1, 1, 2], 'c': [5], 'b': [2, 3]}

in Python?

Edit: I was looking for a functional solution (using only 1 expression).

asked Oct 25, 2016 at 8:33

2

You may use the collections.defaultdict(). Alternatively, in case you do not want to import collections , you may achieve the same behavior with normal dict using dict.setdefault() as:

>>> my_dict = {}
>>> l = [('a', 1), ('b', 2), ('a', 1), ('a', 2), ('c', 5), ('b', 3)]
>>> for k, v in l:
...     my_dict.setdefault(k, []).append(v)
...
>>> my_dict
{'a': [1, 1, 2], 'c': [5], 'b': [2, 3]}

answered Oct 25, 2016 at 8:57

Moinuddin QuadriMoinuddin Quadri

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After defining

from itertools import *

def group(iterable, key, value = lambda x: x):
    return dict((k, list(map(value, values))) for k, values in groupby(sorted(iterable, key = key), key))

use group(l, key = lambda x: x[0], value = lambda x: x[1])).

answered Oct 25, 2016 at 8:33

Using defaultdict from the collections module. You can iterate the list and extract the (key, value) pairs from the tuples in the list and build your dictionary.

from collections import defaultdict

l = [('a', 1), ('b', 2), ('a', 1), ('a', 2), ('c', 5), ('b', 3)]

def dict_my_list(lst):
    d = defaultdict(list)
    for key, value in lst:
        d[key].append(value)
    return d

print dict_my_list(l)  # {'a': [1, 1, 2], 'c': [5], 'b': [2, 3]}

answered Oct 25, 2016 at 8:40

Inbar RoseInbar Rose

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Here is a simple solution to your problem using dict()

l = [('a', 1), ('b', 2), ('a', 1), ('a', 2), ('c', 5), ('b', 3)]
ll = dict()
for key,value in l:
    if key in ll:
        ll[key].append(value)
    else:
        ll[key] = list()
        ll[key].append(value)

print ll  # {'a': [1, 1, 2], 'c': [5], 'b': [2, 3]}

answered Oct 25, 2016 at 8:48

User_TargaryenUser_Targaryen

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The itertools function in Python provides an efficient way for looping lists, tuples and dictionaries. The itertools.groupby function in itertools will be applied in this tutorial to group a list of dictionaries by a particular key.

To illustrate how this works, we will look at a list of students info (in dictionaries) and try to group these by “class” key such…

df.groupby('Column1')['Column3'].apply(list).to_dict()

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