Sorting identical keys in a dictionary

Question

I just started learning python and was wondering if its possible to pair two identical keys?

example:

my_dict= {1:a,3:b,1:c}

and I want to make a new dict, something like this:

new_dict= {1:{a,c},3:{b}}

thanks for the help

The original dict is illegal (try it) but your required outcome is possible. You can make each dict entry a list and have it contain the values you want. — Paul Rooney, Nov 25 '16 at 23:31
You should read about hashing. And how it makes identical keys impossible. — Eli Korvigo, Nov 25 '16 at 23:32
You cannot have duplicate keys in a dictionary in the first place. — Christian Dean, Nov 25 '16 at 23:32
Your input would need to be different something like `[(1,'a'),(3,'b'),(1,'c')]` and your output `{1: ['a','c'], 3: ['b']}`. — Paul Rooney, Nov 25 '16 at 23:47
See [here](http://stackoverflow.com/questions/3199171/append-multiple-values-for-one-key-in-python-dictionary) for ideas on how to do that. — Paul Rooney, Nov 25 '16 at 23:54

score 1 · Accepted Answer · answered Nov 25 '16 at 23:28

1

You can't have identical keys, that is the definition of dictionary. As soon as you add again the same key, the previous entry is deleted

answered Nov 25 '16 at 23:28

blue_note

27,712
9
72
90

score 1 · Answer 2 · answered Nov 25 '16 at 23:29

1

You cannot have repeated dictionary keys in python.

answered Nov 25 '16 at 23:29

rabbit

21
4

score 1 · Answer 3 · answered Nov 26 '16 at 00:33

From what I understand, you're trying to combine the two dictionaries. Given that you cannot have same keys in a dictionary, I'll suppose you have two distinct dictionaries you'd like to combine to obtain your combination.

Ex:

dic1 = {'a': 1, 'b': 2, 'c': 3}

dic2 = {'c': 4, 'd': 5, 'e': 6}

And the combination would produce:

{'a': {1}, 'b': {2}, 'c':{3, 4}, 'd': {4}, 'e': {6}}

You could use this Example:

from itertools import chain
from collections import defaultdict

dic1 = {'A': 1, 'B': 2, 'C': 3}
dic2 = {'C': 4, 'D': 5, 'E': 6}

dic = defaultdict(set)

for k, v in chain(dic1.items(), dic2.items()):
    dic[k].add(v)

print(dict(dic))

Something to note:

'dic' is not exactly a dict(), it's of type 'defaultdict', but you can do dict(dic) to make it so.
You could instead of using set, use list in the defaultdict() argument and have a dictionary as so:

{'a': [1], 'b': [2], 'c':[3, 4], 'd': [4], 'e': [6]}

But to do so, dic[k].append(v) would be used in the for loop. You add items to sets, append to lists.

Christian Dean · Answer 4 · 2016-11-25T23:42:11.307

Dictionaries in Python, or any hash-able type for that matter, will not allow duplicate keys. If two duplicate keys are found in a dictionary, the key farthest in the dictionary will be persevered. This behavior can be obsevered first hand:

>>> my_dict= {1:'a',3:'b',1:'c'}
>>> my_dict
{1: 'c', 3: 'b'}
>>> my_dict= {1:'c',3:'b',1:'a'}
>>> my_dict
{1: 'a', 3: 'b'}
>>>

The Python documentation for dict()s touches on this matter:

It is best to think of a dictionary as an unordered set of key: value pairs, with the requirement that the keys are unique (within one dictionary).[...].

_{(emphasis mine)}

Sorting identical keys in a dictionary

4 Answers4