How Is the Dictionaries Data Structure Implemented in Python?


Angela Bailey

Python provides a powerful data structure called dictionaries that allows you to store and retrieve key-value pairs efficiently. In this article, we will explore how dictionaries are implemented in Python and understand the underlying mechanisms that make them so versatile.

What is a Dictionary?

A dictionary is an unordered collection of elements, where each element is stored as a key-value pair. The keys in a dictionary are unique, and they can be of any immutable data type such as strings, numbers, or tuples. The values, on the other hand, can be of any data type – mutable or immutable.

Creating a Dictionary

In Python, you can create a dictionary using curly braces {}. Let’s say we want to create a dictionary to store the ages of different people:

ages = {'John': 25, 'Jane': 30, 'Tom': 35}

Here, ‘John’, ‘Jane’, and ‘Tom’ are the keys, and their respective ages 25, 30, and 35 are the corresponding values.

Accessing Values in a Dictionary

To access the value associated with a particular key in a dictionary, you can use square brackets [] with the key. For example:


This will output `25`.

If you try to access a key that doesn’t exist in the dictionary, Python will raise a KeyError. To avoid this error, you can use the `get()` method instead:


This will also output `30`.

Modifying Values in a Dictionary

You can update or modify values in a dictionary by assigning a new value to an existing key:

ages['Tom'] = 40

This will output `40`.

If the key doesn’t exist, assigning a value to it will create a new key-value pair in the dictionary.

Iterating Over a Dictionary

You can iterate over a dictionary using a for loop. By default, the loop iterates over the keys of the dictionary:

for key in ages:
    print(key, ages[key])

This will output:

John 25
Jane 30
Tom 40

If you want to iterate over both keys and values, you can use the `items()` method:

for key, value in ages.items():
    print(key, value)

This will produce the same output as before.

The Underlying Implementation of Dictionaries in Python

Dictionaries in Python are implemented using a hash table data structure. A hash table is an array of items where each item consists of a key-value pair and an index computed using a hash function.

When you add an item to a dictionary, Python computes the hash value of the key and uses it as an index to store the item in the array. If two keys have the same hash value (a collision), Python uses an algorithm called open addressing or chaining to resolve it.

The beauty of dictionaries lies in their efficiency. The time complexity for operations like accessing, inserting, and deleting items from a dictionary is O(1) on average. This makes dictionaries perfect for scenarios where fast lookup based on keys is required.

In Conclusion

Dictionaries are a fundamental data structure in Python that allows you to store and retrieve data efficiently using key-value pairs. They are implemented using hash tables, which provide fast access times regardless of the size of the dictionary.

With the knowledge gained from this article, you should now have a better understanding of how dictionaries work in Python and be able to leverage their power in your own programs. So go ahead and start using dictionaries to organize your data effectively!

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