defaultdict in Python

A defaultdict is a dictionary that creates a starting value the first time you use a missing key. This removes the "is the key there yet?" check from grouping and counting code.

The program

from collections import defaultdict

books = [
    ("Fiction", "Dune"),
    ("Science", "Cosmos"),
    ("Fiction", "Emma"),
    ("History", "SPQR"),
    ("Science", "The Selfish Gene"),
]

by_genre = defaultdict(list)
for genre, title in books:
    by_genre[genre].append(title)

for genre, titles in by_genre.items():
    print(f"{genre}: {', '.join(titles)}")

Fiction: Dune, Emma
Science: Cosmos, The Selfish Gene
History: SPQR

How it works

• defaultdict(list) takes a function that makes the starting value. Here it is list, so a missing key starts as an empty list [].
• The first time by_genre["Fiction"] is used, the key is created with [], and then .append("Dune") adds to it.
• With a normal dictionary, by_genre[genre].append(title) would raise a KeyError for the first book of each genre.

Counting with defaultdict(int)

int() returns 0, so defaultdict(int) starts every count at 0:

from collections import defaultdict

votes = ["red", "blue", "red", "green", "red", "blue"]

tally = defaultdict(int)
for colour in votes:
    tally[colour] += 1

print(dict(tally))
print(tally["purple"])
print(dict(tally))

{'red': 3, 'blue': 2, 'green': 1}
0
{'red': 3, 'blue': 2, 'green': 1, 'purple': 0}

Notice the last line: just reading a missing key adds it. To check for a key without adding it, use "purple" in tally or tally.get("purple").