collectionsOrderedDict

collections.OrderedDict is a dict-like class from Python's standard library (the collections module) that records the order in which keys are inserted. Iterating over an OrderedDict yields items in that insertion order, even after updates or deletions. Although Python's built-in dict now preserves insertion order, OrderedDict provides additional methods for reordering and for modeling ordered data structures.

Internally, OrderedDict uses a doubly linked list to maintain element order while a separate dictionary provides fast key lookups. This design yields efficient operations and keeps a stable iteration order. The API includes move_to_end(key, last=True) to relocate an existing key to the end (or to the beginning if last is False) and popitem(last=True) to remove and return a pair from either end. The update method can insert new keys, appending them to the end.

Typical usage is to create a cache or otherwise manage items in a defined order. For example: from collections import OrderedDict; od = OrderedDict([('a', 1), ('b', 2)]); od['c'] = 3; od.move_to_end('b'); for k, v in od.items(): print(k, v).

With Python 3.7 and later, plain dicts preserve insertion order, reducing the need for OrderedDict in many cases. OrderedDict remains useful when you require explicit control over item order with methods like move_to_end or when you need a predictable representation of a mapping that is not provided by dict. Conversions between the two are straightforward: dict(od) or OrderedDict(od).