When trying to create a dictionary from two lists, some elements are lost

Question

I have two lists:

list_1 = ['2', '1', '1', '2', '1', '2', '1', '5', '4', '3', '2', '1', '3', '2', '1']
list_2 = ['az', 'z', 'c', 'bo', 'o', 'bo', 'o', 'beggh', 'eggh', 'ggh', 'gh', 'h', 'akl', 'kl', 'l']

The number of strings inside the two lists is the same. I want to create a dictionary from the two lists, so I try:

new_dict = dict(zip(list_1, list_2))

I expect something like:

{
  '2' : 'az',
  '1' : 'z',
  '1' : 'c',
  ....


}

I also expect that the number of key-value pairs in the dictionary is the same as the number of strings in either list_1 or list_2. However, when I try to print the dictionary out, it gives:

{'2': 'kl', '1': 'l', '5': 'beggh', '4': 'eggh', '3': 'akl'}

The number of key-value pairs is only 5, which is obviously less than the number of strings in either list_1 or list_2. What has gone wrong?

You can't have duplicate keys in a dictionary. You probably want a dictionary of lists if you don't want to lose data — user3483203, Sep 13 '18 at 17:55
Dictionary keys must be unique, that's part of the point of dicts. — Daniel Roseman, Sep 13 '18 at 17:55
As @user3483203 said, you can't have duplicate keys. If you need to pair up those values make a list of tuples or a list of lists. — Lucas Wieloch, Sep 13 '18 at 17:56
Dictionary comprehension won't gracefully handle duplicate keys. — kindall, Sep 13 '18 at 18:14

score 3 · Accepted Answer · answered Sep 13 '18 at 18:01

You cannot have a dictionary with duplicated keys, one solution would be to group values by key, here an example using itertools.groupby

from itertools import groupby
list_1 = ['2', '1', '1', '2', '1', '2', '1', '5', '4', '3', '2', '1', '3', '2', '1']
list_2 = ['az', 'z', 'c', 'bo', 'o', 'bo', 'o', 'beggh', 'eggh', 'ggh', 'gh', 'h', 'akl', 'kl', 'l']

new_dict = {
    k : list(map(lambda x: x[1], v)) for k, v in groupby(sorted(zip(list_1, list_2)), key=lambda x: x[0])
}
print(new_dict)

Live example

Or simply use a defaultdict

from collections import defaultdict
new_dict = defaultdict(list)
for k, v in zip(list_1, list_2):
    new_dict[k].append(v)

When trying to create a dictionary from two lists, some elements are lost

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