What is the `collections.defaultdict` class?

Question

Accepted Answer

**defaultdict** — dictionary that provides default value for missing keys.

```python
from collections import defaultdict

# Count occurrences
word_count = defaultdict(int)
for word in ["apple", "banana", "apple", "cherry"]:
    word_count[word] += 1
print(dict(word_count))  # {'apple': 2, 'banana': 1, 'cherry': 1}

# Group items
groups = defaultdict(list)
for item in [(1, "a"), (1, "b"), (2, "c")]:
    groups[item[0]].append(item[1])
print(dict(groups))  # {1: ['a', 'b'], 2: ['c']}

# Nested dictionary
nested = defaultdict(lambda: defaultdict(int))
nested["users"]["count"] += 1
```

**vs dict.setdefault():**
- defaultdict: cleaner syntax, factory function.
- setdefault: works on existing dicts.

Python: What is the `collections.defaultdict` class?