[Python] Python reverse dictionary key value reference relationships

If I have a following dictionary:

{"1": {"2", "3"}, "2": {"3", "4"}, "3": {"2", "4"}}


In this dictionary, the key references the values in the set.

How do I construct a new dictionary which defines the references to the values above:

{"1": {}, "2": {"1", "3"}, "3": {"1", "2"}, "4": {"2", "3"}}


This is what I currently do:

for k,v in data.items():
if len(v) > 0:
for p in v:
references[p].add(k)
else:
for p,v in references.items():
v.add(k)


Any better / optimized /pythonic way of doing it?

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