Using dict.setdefault in Python

Posted on by Jerry Wang
Photo by Christina Morillo on Pexels.com

For years, I’ve been using the following snippet in the solution for one of my interview questions:

anagrams = dict()

with open(WORDS_PATH) as f:
    for line in f:
    key = sort(line.strip())
    if key not in anagrams:
        anagrams[key] = list()
        anagrams[key].append(line.strip())
    else:
        anagrams[key].append(line.strip())

Recently, I learned to use the dict.setdefault function to further optimize it, and the end result looks like the following:

anagrams = dict()

with open(WORDS_PATH) as f:
    for line in f:
        key = sort(line.strip())
        anagrams.setdefault(key, []).append(line.strip())

Get the list of anagrams for key, or set it to [] if not found; setdefault returns the value, so it can be updated without requiring a second search. In other words, the end result of this line…

anagrams.setdefault(key, []).append(line.strip())

…is the same as running…

if key not in anagrams:
    anagrams[key] = list()
    anagrams[key].append(line.strip())
else:
    anagrams[key].append(line.strip())

…except that the latter code performs at least two searches for key – three if it’s not found – while setdefault does it all with a single lookup.

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