collections

Container datatypes beyond the built-ins: ordered/default dicts, counters, deques, named tuples, chain maps, and user-extensible bases. The collections.abc submodule houses the abstract base classes for the container protocols.

Source-of-record: Lib/collections/__init__.py, Modules/_collectionsmodule.c, collections docs.

Counter

Counter(iterable_or_mapping=None, **kwargs)

Multiset / frequency map. Subclass of dict.

Method	Behaviour
c.elements()	Iterator of elements repeated by count.
c.most_common(n=None)	[(elem, count), ...] sorted descending.
c.subtract(other)	Element-wise subtraction; can go negative.
c.update(other)	Element-wise addition.
c.total()	Sum of counts (3.10+).
c + c2, c - c2	Multiset add / subtract (drops non-positive).
c | c2, c & c2	Multiset union (max) / intersection (min).
+c, -c	Drop non-positive / non-negative counts.

OrderedDict

dict subclass that preserved insertion order even before 3.7 made the plain dict order-stable. Still distinct because of:

Method	Behaviour
od.move_to_end(key, last=True)	Reorder.
od.popitem(last=True)	LIFO by default; last=False for FIFO.
Equality	Order-sensitive between two OrderedDicts.

defaultdict

defaultdict(default_factory=None, /, *args, **kwargs)

Provides a default value via default_factory() on missing keys.

Attribute	Meaning
dd.default_factory	Zero-arg callable; missing keys call it.
dd[k]	Inserts default_factory() if missing.
dd.__missing__(k)	Hook the lookup uses.

deque

deque(iterable=(), maxlen=None)

Doubly-linked list-like container with O(1) appends/pops at both ends.

Method	Behaviour
append(x) / appendleft(x)	Push.
pop() / popleft()	Pop.
extend(it) / extendleft(it)	Bulk push (note: extendleft reverses).
rotate(n=1)	Rotate right (or left if negative).
clear(), copy(), count(x), index(x[, start[, stop]]), insert(i, x), remove(x), reverse()	Stdlib operations.
maxlen	Read-only cap.

ChainMap

ChainMap(*maps)

Composes several mappings into one view. Writes go to the first map; reads cascade through the chain.

Method / attr	Role
maps	List of underlying mappings.
new_child(m=None)	Push a fresh mapping (or m) onto the chain.
parents	View without the first mapping.

namedtuple

collections.namedtuple(typename, field_names, *, rename=False,
                       defaults=None, module=None)

Generates a tuple subclass with named fields.

Generated member	Notes
T._fields	Field names.
T._field_defaults	Defaults dict.
t._asdict()	Dict view.
t._replace(**changes)	Returns a new tuple with fields replaced.
T._make(iterable)	Construct from iterable.

UserList, UserDict, UserString

Subclassable wrappers around list, dict, str. Useful where subclassing the built-in is awkward (mutators may bypass the wrapper).

collections.abc

Re-exports the ABCs from _collections_abc:

Group	ABCs
Hashable	Hashable.
Iterable	Iterable, Iterator, Generator, Reversible, Collection.
Containers	Container, Sized.
Sequence	Sequence, MutableSequence, ByteString (deprecated 3.12).
Set	Set, MutableSet.
Mapping	Mapping, MutableMapping, MappingView, KeysView, ValuesView, ItemsView.
Callable	Callable.
Async	Awaitable, Coroutine, AsyncIterable, AsyncIterator, AsyncGenerator.
Buffer (3.12)	Buffer.

Gopy status

Area	State
Counter, OrderedDict, defaultdict	Complete.
deque	Complete; C-fast path in module/_collections.
ChainMap	Complete.
namedtuple	Complete.
collections.abc ABCs	Complete.

Reference

• CPython 3.14: collections.
• Lib/collections/, Modules/_collectionsmodule.c.
• module/collections/, module/_collections/. gopy port.