Inverting a dictionary of lists in Python | Hugonweb Annotated Link Bibliography

I've needed to do this countless times in particle physics data analysis and other areas.

What I've done before:

data = {'a' : ['alpha', "beta"], 'b' : ["alpha"]}
result = {}
for key in data:
    for value in data[key]:
        try:
            result[value].append(key)
        except KeyError:
            result[value] = [key]

and the simpler solutions suggested in the link:

data = {'a' : ['alpha', "beta"], 'b' : ["alpha"]}
result = {}
for key in data:
    for value in data[key]:
        result.setdefault(value,[]).append(key)

or

from collections import defaultdict

data = {'a' : ['alpha', "beta"], 'b' : ["alpha"]}
result = defaultdict(list)
for key in data:
    for value in data[key]:
        result[value].append(key)

dict.setdefault and collections.defaultdict are new to me and useful!