collections

Small container helpers for counting, grouping, and queues.

Counter

Counter(iterable) returns a plain dictionary of occurrence counts, rather than a CPython Counter instance. Missing keys raise KeyError; use get(key, 0) for a zero default. Methods such as most_common() are not provided.

from collections import Counter, defaultdict, deque

counts = Counter('banana')
assert counts == {'b': 1, 'a': 3, 'n': 2}
assert counts.get('z', 0) == 0

groups = defaultdict(list)
groups['fruit'].append('apple')
assert groups['fruit'] == ['apple']

recent = deque([1, 2], maxlen=3)
recent.append(3)
recent.append(4)
assert list(recent) == [2, 3, 4]
assert recent.popleft() == 2

defaultdict

defaultdict(default_factory, *args) calls the factory with no arguments when mapping[key] is missing, stores the result, and returns it. Provide a callable factory such as list or int. get() does not create an entry. copy() preserves the factory.

deque

deque(iterable=None, maxlen=None) is a double-ended queue. A bounded queue requires a positive maxlen; appending to a full queue removes an item from the opposite end.

Operations	Methods
Add items	`append`, `appendleft`, `extend`, `extendleft`
Remove items	`pop`, `popleft`, `clear`
Inspect/copy	`count`, `copy`, `maxlen`, `len()`, iteration, membership, equality
Rotate	`rotate(n=1)`: positive moves right, negative moves left

extendleft() inserts each element at the left, reversing the input order. Popping an empty queue raises IndexError. Indexing and slicing are not implemented; convert to a list when needed.

Implementation: collections.py.