How Does Python Implement Dictionaries Using Hash Tables?

Python Dictionary Implementation: Understanding the Hash Table Structure

Python's built-in dictionary is implemented as a hash table, a data structure designed for efficient insertion, deletion, and retrieval based on a key-value pair.

Components of a Hash Table

Each slot in the hash table contains three values: the hash of the key, the key itself, and the associated value. When a new key-value pair is added, the hash of the key is used to determine the initial slot to insert into. However, hash collisions may occur when two keys have the same hash.

Open Addressing and Hash Collisions

Python dictionaries utilize open addressing to resolve hash collisions. This means that when a collision occurs, the key-value pair is inserted into the first available empty slot using a probing technique.

Random Probing

When probing for an empty slot, Python uses random probing, which selects the next slot based on a pseudo-random algorithm. This helps evenly distribute key-value pairs throughout the hash table, reducing the likelihood of performance degradation caused by collisions.

Resizing and Threshold

The hash table is initially sized with 8 slots. When the number of entries reaches two-thirds of the table's capacity, the table is resized to twice its original size to maintain efficient lookups.

Note:

The implementation described applies to Python versions prior to 3.6. For Python 3.6 and later, the dict implementation utilizes a combination of hash tables and linked lists to improve performance and memory usage, known as the "dict-of-dicts" approach.

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