Compare two dict and change keys in second if keys exists as sub-value in first

Question

Need help remapping the keys for d2 if they match the value for id in d1. In case there are keys that are unmatched, drop them.

d1={'A':{'id':'a'},'B':{'id':'b'}}
d2={'a':1,'b':2, 'c':3}

Expected output:

{'A': 1, 'B': 2}

Answer 1

This is probably what you're after

d1={'A':{'id':'a'},'B':{'id':'b'}}
d2={'a':123,'b':123}

for key, id_map in d1.copy().items():  # view over soft copy won't mutate
    try:
        d1[key] = d2[id_map["id"]]
    except KeyError:
        pass

>>> d1
{'A': 123, 'B': 123}

However, unmapped values will remain the same (ie if there is C, but no id in its mapping, or no c in d2..)

>>> d1={'A':{'id':'a'},'B':{'id':'b'},'C':{'id':'c'}}
[..]
>>> d1
{'A': 123, 'B': 123, 'C': {'id': 'c'}}

If you want to discard unmapped values, you could do so here with del d1[key] in the except or simply create a new dict to pack the keys and values into (never added if KeyError is raised)
Alternatively, if this is an error (ie. C must exist), you can simply let KeyError raise to the caller and have it deal with the consequences

Answer 2

As long as each 'val' appears at most once in d1's subdictionaries (i.e. the key: {'id': val} pairs in d1), you can iterate over d1's items like so:

d1 = {'A': {'id': 'a'}, 'B': {'id': 'b'}, 'C': {'id': 'c'}}
d2 = {'a': 123, 'b': 12, 'd': 8}

d3 = {k: d2[v['id']] for k, v in d1.items() if v['id'] in d2}
print(d3)

>>> {'A': 123, 'B': 12}

Since you need to check all key-value pairs in d1, this is the best you can do in terms of time and space efficiency.

However, if you will be 're-keying' many d2 dictionaries with the same d1, it may be faster to build a reverse-index to reuse:

reverse_mapping = {v['id']: k for k, v in d1.items()}
def rekey(dict_to_rekey, reverse_map):
    return {reverse_map[k]: dict_to_rekey[k] 
            for k in dict_to_rekey.keys() & reverse_map.keys()}

d3 = rekey(d2, reverse_mapping)