defaultdict.py

# defaultdict — a dictionary that returns a default value for missing keys.
#
# Normally, accessing a missing key raises a KeyError.
# defaultdict takes a factory function; when a missing key is accessed,
# it calls the factory to create a default value automatically.

from collections import defaultdict


# --- Standard dict — KeyError on missing key ---

d = {1: 'apple', 2: 'banana', 3: 'cherry'}
print('Standard dict:')
print(d[1])

try:
    print(d[4])
except KeyError:
    print('KeyError: key 4 not found')


# --- defaultdict with None default ---

print('\ndefaultdict (default None):')

dd = defaultdict(lambda: None)
dd['a'] = 1
dd['b'] = 2

print(dd['a'])    # 1
print(dd['b'])    # 2
print(dd['c'])    # None — key created automatically, no error
print(dict(dd))   # {'a': 1, 'b': 2, 'c': None}


# --- defaultdict with list default ---
# Useful for grouping values by key without checking if the key exists.

print('\ndefaultdict (default list) — grouping words by first letter:')

words = ['apple', 'banana', 'avocado', 'blueberry', 'cherry', 'apricot']
grouped = defaultdict(list)

for word in words:
    grouped[word[0]].append(word)

for letter, group in sorted(grouped.items()):
    print(f'  {letter}: {group}')


# --- defaultdict with int default ---
# Useful for counting without initialising counts to 0.

print('\ndefaultdict (default int) — word frequency count:')

sentence = 'the cat sat on the mat the cat'
counts = defaultdict(int)

for word in sentence.split():
    counts[word] += 1

for word, count in sorted(counts.items()):
    print(f'  {word}: {count}')