How to create a dictionary with equal keys?

Question

I have a problem with keys in a dict in python.I have a list of lists:

x=[['A','B','C','D'],['A','B','E','F'],['A','B','G','H']]

This is my code:

{(tuple(t[:2])):t[2:] for t in x}

And this is my output:

{('A', 'B'): ['G', 'H']}

The code takes only the last key / value, because there are identical keys.

The output should be:

{('A', 'B'):[['C','D']['E','F'],['G','H']]}

I can't import libraries.

Make a `for` loop and add the values to the list (in the dict) one-by-one. This is not really a job for a dict comprehension. — zvone, Dec 10 '20 at 22:44
If you want another one-liner: `{(tuple(t[:2])):[s[2:] for s in x if s[:2] == t[:2]] for t in x}` — fafl, Dec 10 '20 at 22:46
Also: [Is there a way to preserve duplicate keys in python dictionary](https://stackoverflow.com/q/11141383/7851470) — Georgy, Dec 13 '20 at 10:52

Dani Mesejo · Accepted Answer · 2020-12-10T22:52:41.137

You could iterate over the elements and put them in a dictionary according to the key:

x = [['A', 'B', 'C', 'D'], ['A', 'B', 'E', 'F'], ['A', 'B', 'G', 'H']]

res = {}
for f, s, *tail in x:
    if (f, s) in res:
        res[(f, s)].append(tail)
    else:
        res[(f, s)] = [tail]

print(res)

Output

{('A', 'B'): [['C', 'D'], ['E', 'F'], ['G', 'H']]}

If the list are sorted by the first two elements, you could use itertools.groupby:

from itertools import groupby
from operator import itemgetter

first_two = itemgetter(0, 1)

x = [['A', 'B', 'C', 'D'], ['A', 'B', 'E', 'F'], ['A', 'B', 'G', 'H']]

res = {k : [e[2:] for e in group] for k, group in groupby(x, first_two)}
print(res)

Output

{('A', 'B'): [['C', 'D'], ['E', 'F'], ['G', 'H']]}

score 4 · Answer 2 · answered Dec 10 '20 at 22:45

Use a defaultdict and loop through:

from collections import defaultdict

vals = defaultdict(list)
x=[['A','B','C','D'],['A','B','E','F'],['A','B','G','H']]
for val in x:
    vals[tuple(val[:2])].append(val[2:])

# get rid of default value
vals = dict(vals)

How to create a dictionary with equal keys?

2 Answers2