Merging 2 dictionaries and saving translation dictionary for the overridden values

Question

I merge 2 dictionaries, say d1 and d2 by:

d1.update(d2)

i.e I have:

d1 = {1:'a', 2:'b', 3:'c'}
d2 = {1:'a', 2:'g', 4:'x'}

then after d1.update(d2) I have:

{1: 'a', 2: 'g', 3: 'c', 4: 'x'}

So the original 2:'b' vanished so I'd like to have the translation 'b':'g' somehow, meaning the output will be the following dictionary:

{'b':'g'}

Is there any simple code to keep the overridden values?

Possible duplicate of [Merge dictionaries retaining values for duplicate keys](https://stackoverflow.com/questions/24745181/merge-dictionaries-retaining-values-for-duplicate-keys) — Christian W., Aug 20 '18 at 07:16
@Aran-Fey In my example I'd like to output a dictionary which contains {'b':'g'} — Codevan, Aug 20 '18 at 07:21

score 4 · Accepted Answer · answered Aug 20 '18 at 07:26

If {"b":"g} is what you truly wants, you can first use dict comprehension to create it before the update:

d1 = {1:'a', 2:'b', 3:'c'}
d2 = {1:'a', 2:'g', 4:'x'}

ot = {v : d2[k] for k,v in d1.items() if k in d2 and v != d2[k]}
print (ot)

d1.update(d2)
print (d1)

Result:

{'b': 'g'}
{1: 'a', 2: 'g', 3: 'c', 4: 'x'}

score 0 · Answer 2 · answered Aug 20 '18 at 07:27

Find the keys that exist in both dictionaries
Create a new dictionary mapping the values from d1 to the values from d2
Remove those key:value pairs where the key and the value are identical

d1 = {1:'a', 2:'b', 3:'c'}
d2 = {1:'a', 2:'g', 4:'x'}

shared_keys = d1.keys() & d2
mapping = {d1[key]: d2[key] for key in shared_keys}
mapping = {k: v for k, v in mapping.items() if k != v}

print(mapping)  # output: {'b': 'g'}

Merging 2 dictionaries and saving translation dictionary for the overridden values

2 Answers2