DefaultDict

Introduction

DefaultDict provides default values for missing keys, eliminating key existence checks.

Basic Usage

from collections import defaultdict

# List as default factory
dd_list = defaultdict(list)
dd_list["fruits"].append("apple")
print(dd_list["fruits"])  # ['apple']
print(dd_list["vegetables"])  # [] - empty list created

# Integer as default (counter)
dd_int = defaultdict(int)
dd_int["apples"] += 1
print(dd_int["apples"])  # 1
print(dd_int["oranges"])  # 0

Grouping with DefaultDict

from collections import defaultdict

fruits = ["apple", "banana", "apple", "cherry", "banana", "apple"]

# Group by fruit
grouped = defaultdict(list)
for fruit in fruits:
    grouped[fruit].append(fruit)

# Result: {'apple': ['apple', 'apple', 'apple'], ...}

Custom Factory

from collections import defaultdict

def default_dict_factory():
    return {"count": 0, "items": []}

dd_custom = defaultdict(default_dict_factory)
dd_custom["key"]["count"] += 1

Practice Problems

1. Count word frequencies in text
2. Group items by category
3. Create nested default dictionary
4. Use set as default factory
5. Build adjacency list for graph