How to Use defaultdict in Python: Simple Guide with Examples

Use defaultdict from the collections module to create a dictionary that provides a default value for missing keys automatically. Initialize it with a function that returns the default value, so you avoid KeyError when accessing keys that don't exist.

Syntax

The defaultdict is created by importing it from the collections module and initializing it with a default factory function. This function returns the default value for any missing key.

• defaultdict(default_factory): Creates a dictionary where missing keys return default_factory() value.
• default_factory: A function like int, list, or a custom function that returns the default value.

python

from collections import defaultdict

d = defaultdict(int)  # int() returns 0 as default

print(d['missing_key'])  # prints 0

Output

0

Example

This example shows how to count the frequency of letters in a word using defaultdict. It automatically starts counts at zero, so you don't need to check if the letter is already in the dictionary.

python

from collections import defaultdict

word = 'banana'
letter_count = defaultdict(int)

for letter in word:
    letter_count[letter] += 1

print(dict(letter_count))

Output

{'b': 1, 'a': 3, 'n': 2}

Common Pitfalls

One common mistake is to use a regular dictionary and try to access or increment a key that doesn't exist, which causes a KeyError. Another is to forget that defaultdict requires a callable for the default factory, not a value.

Wrong way:

counts = {}
counts['a'] += 1  # KeyError because 'a' is missing

Right way with defaultdict:

python

from collections import defaultdict

counts = defaultdict(int)
counts['a'] += 1
print(counts['a'])

Output

1

Quick Reference

Feature	Description	Example
default_factory	Function returning default value for missing keys	int, list, lambda: 'N/A'
Access missing key	Returns default value instead of KeyError	d['missing'] returns 0 if int() used
Increment counts	Easily count items without key checks	d[key] += 1
Custom default	Use any function for default values	defaultdict(lambda: 'unknown')

Key Takeaways

Use defaultdict to avoid KeyError by providing default values automatically.

Initialize defaultdict with a function like int or list to set default values.

defaultdict is perfect for counting or grouping data without manual key checks.

Remember default_factory must be a callable, not a direct value.

Convert defaultdict to dict if you want a regular dictionary output.