What Data Structure Is Used for Python Dictionary?


Scott Campbell

A Python dictionary is a built-in data structure that allows you to store and retrieve data in key-value pairs. It is a powerful tool for organizing and manipulating data efficiently. In this article, we will explore the underlying data structure used for Python dictionaries.

Hash Tables:
One of the most commonly used data structures for implementing Python dictionaries is a hash table. A hash table, also known as a hash map, is an array-based data structure that uses a hash function to compute an index into an array of buckets or slots, where each slot can store multiple key-value pairs.

Hash Functions:
A crucial component of a hash table is the hash function. It takes an input (in this case, the dictionary key) and computes a unique value called the hash code. The hash code determines the index in the array where the key-value pair will be stored.

Since there are typically more possible keys than available slots in an array, collisions may occur when two different keys produce the same hash code. To handle collisions, most modern programming languages use collision resolution techniques such as chaining or open addressing.

Chaining involves storing multiple key-value pairs in each slot of the array. Each slot contains a linked list or any other suitable data structure to handle multiple entries with the same index.

Open Addressing:
Open addressing takes a different approach by finding another available slot within the array to store the collided item. This can be done using techniques like linear probing (checking adjacent slots) or quadratic probing (checking slots with quadratic intervals).

Python’s Dictionary Implementation:
Python’s dictionary implementation uses a combination of these techniques. When you create a dictionary in Python, it allocates memory for an empty table with some initial number of slots. As you add key-value pairs to the dictionary, it dynamically resizes the table when it reaches a certain threshold to maintain efficient operations.

Internal Structure:
Internally, Python’s dictionary consists of three main components:

  • Hash Table: The hash table is an array of slots, where each slot can store one or more key-value pairs. It is the primary data structure for storing and retrieving items.
  • Keys: Python dictionary keys are unique and immutable objects (such as strings, numbers, or tuples) that determine the index of the slot using the hash function.
  • Values: Values can be any object or data type and are associated with their respective keys in the hash table.

Efficiency Considerations:
Python dictionaries provide fast access to items by key, with an average time complexity of O(1) for retrieval, insertion, and deletion operations. However, in worst-case scenarios with many collisions, these operations may degrade to O(n), where n is the total number of items in the dictionary.

In Conclusion

Python dictionaries use hash tables as their underlying data structure for efficient storage and retrieval of key-value pairs. Understanding how dictionaries work internally can help you optimize your code and make informed decisions while working with large datasets. Remember to choose appropriate keys that produce well-distributed hash codes to minimize collisions for optimal performance.

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