collections.defaultdict

Create a dictionary that adds a missing key with a default value when you access it.

Python 3.13
collections.defaultdict([default_factory])

Parameters

default_factoryCallable | Noneoptional: A function without arguments that creates the default value, such as list or int; with None, a missing key raises KeyError.

Return value

A defaultdict object, a dictionary subclass.

Examples

Grouping words by first letter

Python 3.13
from collections import defaultdict

groups = defaultdict(list)
for word in ['apple', 'avocado', 'banana']:
    groups[word[0]].append(word)
print(dict(groups))
{'a': ['apple', 'avocado'], 'b': ['banana']}

Counting letters with a default of 0

Python 3.13
from collections import defaultdict

counts = defaultdict(int)
for letter in 'hello':
    counts[letter] += 1
print(counts['l'], counts['z'])
2 0

Details

Pass the function itself without parentheses: defaultdict(list), not defaultdict([]). A key is added only on access with d[key]; get() and in do not add it.