Exchange a key and one of its values in a dictionary

Question

I have a small example of exchanging a key and one of its values, but I did not make it work:

dict1 = {1: ['BB', 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}
print(dict1)

for key in dict1:
    for value in dict1[key]:
        if value == 'BB':
            temp = key
            key = value            
            value = temp

print(dict1)

Current output:

{1: ['BB', 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}

Desired output:

{BB: [1, 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}

I have used the temp to exchange the value and key, but why does the output does not change (keeps as same as the original dict1)?

The desired output doesn't make sense -- you can't have duplicate keys in a dict. `list1` is really a dict. I don't understand the logic here. Please clarify. — ggorlen, Jul 05 '21 at 20:39
I recommend [Python visualization](http://pythontutor.com/visualize.html#mode=display) and [Python names](https://nedbatchelder.com/text/names.html) to help you understand. — jarmod, Jul 05 '21 at 20:53
I think the question is clear, but it requires some amount of explanation of how Python fundamentally works as an answer. — mkrieger1, Jul 05 '21 at 20:56
A short answer would be: `key` and `value` are new variables here (i.e. they are copies of the original `key` and `value`). — Jānis Š., Jul 05 '21 at 21:40
[5.5. Dictionaries- It is best to think of a dictionary as a set of key: value pairs, with the requirement that the keys are unique (within one dictionary)](https://docs.python.org/3/tutorial/datastructures.html#:~:text=with%20the%20requirement%20that%20the%20keys%20are%20unique%20(within%20one%20dictionary)) — Giorgos Xou, Jul 05 '21 at 21:48
[```No, each key in a dictionary should be unique. You can’t have two keys with the same value. Attempting to use the same key again will just overwrite the previous value stored. If a key needs to store multiple values, then the value associated with the key should be a list or another dictionary.```](https://discuss.codecademy.com/t/can-a-dictionary-have-two-keys-of-the-same-value/351465) — Giorgos Xou, Jul 05 '21 at 21:53
@Giorgos Xou thanks Xou, I had made a mistake on this point, I have just updated the desired output. My intention is simply exchange the value of a key and one of its value:) — Melina, Jul 06 '21 at 07:45
@Jānis Š. Thanks, I think you get the point of the problem. If `key` and `value` are copies of the original `key` and `value`, how can I exchange the ´real key and value´ though? — Melina, Jul 06 '21 at 07:51
C/C++ approach is impossible in your case in Python. You cannot change the key - only add a new one or delete an existing one. To replace an item in the list, you can either create a new list and assign it to a key or use _ctypes (see an example here by martineau (the last answer): https://stackoverflow.com/questions/44205017/how-to-reference-a-dict-object). — Jānis Š., Jul 06 '21 at 10:15

Giorgos Xou · Answer 1 · 2021-07-06T15:41:57.057

A Solution?

Here some kind of a solution? (not as i would desire it to be but it does the job) :

dict1 = {1: ['BB', 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}

for key in list(dict1):
    for i, value in enumerate(dict1[key]):
        if value == 'BB':
            dict1[value] = dict1.pop(key)
            dict1[value][i] = key

print(dict1)

{2: ['FF', 'HH', 'GG'], 'BB': [1, 'CC', 'DD']}

^{^^^⚠️ The sequence is changed ⚠️}

References:

You can’t use for-in loop to modify a list because the iteration variable, (item in your example), is only holding the value from your list and not directly pointing to that particular list item...

How to reference a dict object?

score 0 · Accepted Answer · answered Jul 06 '21 at 08:07

a simple homemade solution, create a new result dict is better than change old dict.

code:

dict1 = {1: ['BB', 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}
result = {}
for k,v in dict1.items():
    if "BB" in v:
        v.remove("BB")
        result["BB"] = [k] + v
    else:
        result[k] = v
print(result)

result:

{'BB': [1, 'CC', 'DD'], 2: ['FF', 'HH', 'GG']}

Exchange a key and one of its values in a dictionary

2 Answers2

A Solution?

References: