Lib/collections/__init__.py

cpython 3.14 @ ab2d84fe1023/Lib/collections/__init__.py ↗

Lib/collections/__init__.py provides the high-level collection types. deque and defaultdict come from _collections (C extension); this file adds OrderedDict, namedtuple, ChainMap, Counter, and the User* wrappers.

Map

Lines	Symbol	Role
1-100	Re-exports from _collections	deque, defaultdict, _count_elements
101-400	OrderedDict	Insertion-order dict backed by a doubly-linked key list
401-700	namedtuple	Code-generates a tuple subclass with named fields
701-900	ChainMap	Read-through view of multiple mappings
901-1100	Counter	Int-valued multiset with arithmetic operations
1101-1200	UserDict, UserList, UserString	Mutable wrappers for subclassing

Reading

OrderedDict doubly-linked key list

OrderedDict maintains a separate doubly-linked list of (prev, next, key) nodes in self.__root. __setitem__ appends to the list; __delitem__ unlinks the node; move_to_end relinks the node to the front or back.

# CPython: Lib/collections/__init__.py:150 OrderedDict.__setitem__
def __setitem__(self, key, value, ...):
    if key not in self:
        self.__root  # link
        ...
    super().__setitem__(key, value)

namedtuple code generation

namedtuple('Point', ['x', 'y']) generates a class body as a string and execs it. The generated class subclasses tuple and provides _fields, _make, _asdict, _replace, and named property slots for each field.

Counter arithmetic

Counter supports +, -, & (intersection), | (union) with other Counter objects. The result omits zero and negative counts from + and -. most_common(n) uses heapq.nlargest for efficiency.

ChainMap.new_child

ChainMap.new_child(m=None) creates a new ChainMap with m (or an empty dict) as the first map, and the existing ChainMap's maps as the rest. This is the recommended way to create nested scopes.

gopy notes

Not yet ported. OrderedDict is needed by many stdlib modules. namedtuple requires exec support. The planned path is module/collections/. Counter can use a dict subclass in Go with arithmetic methods.