How Does Python Guarantee Key Order in Dictionaries?

Maintaining Key/Value Order in Dictionaries

In Python, dictionaries inherently maintain insertion order from version 3.6 onwards. By declaring a dictionary as follows:

The resulting dictionary will preserve the keys in the order they were declared. This behavior differs from earlier Python versions where key order was random.

Under the hood, this order-preservation is achieved through an optimized implementation that utilizes sparse hash tables and arrays to store key-value pairs. The keys are essentially stored in an array in the sequence they are added.

While Python 3.6 considered order-preservation an implementation detail, Python 3.7 elevates this to a language specification. As a result, all Python implementations compatible with version 3.7 and higher must maintain order in their dictionaries. This guarantee extends to reverse iteration as well, which is supported from Python 3.8 onwards.

Despite the introduction of order-preserving dictionaries, the collections.OrderedDict() class from the collections module remains useful in certain scenarios. It offers additional features such as reversibility and the ability to reorder keys using the move_to_end() method.

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