How Does Python Preserve Dictionary Insertion Order?

Preserving Declaration Order in Python Dictionaries

In Python 3.6 and above, dictionaries innately maintain the order in which their key-value pairs are declared. This means that the following dictionary:

d = {'ac': 33, 'gw': 20, 'ap': 102, 'za': 321, 'bs': 10}

Will be preserved in the same order when iterated through or displayed. This behavior is implemented using an integer-indexed sparse hash table, where the integers correspond to key-value pairs stored in an additional array. The latter array ensures insertion order.

In Python 3.7, this order-preserving nature became a language specification, requiring all compatible implementations to observe the order of declared key-value pairs in dictionaries. Additionally, Python 3.8 introduced support for reverse iteration in dictionaries.

While the standard dict type now supports order preservation, you may still consider using the collections.OrderedDict() class. It provides additional functionality, including reversibility (which extends to view objects) and the ability to reorder items using the move_to_end() method.

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