What is the difference between a list and a dictionary in Python?

Question

Jayant Kumar · Accepted Answer

Table of Contents
Introduction
In Python, lists and dictionaries are both versatile and commonly used data structures. While they share some similarities, they have distinct characteristics and are used for different purposes. This guide explains the key differences between lists and dictionaries, including their structure, use cases, and examples.
Key Differences
Data Structure

List: A list is an ordered collection of items. Each item in a list is indexed by an integer, starting from 0. Lists are mutable, meaning you can modify them after their creation.
Dictionary: A dictionary is an unordered collection of key-value pairs. Each item in a dictionary is stored as a pair consisting of a unique key and a corresponding value. Dictionaries are also mutable.

Indexing

List: Lists are indexed by integers. You access elements using their index position.
Example:
fruits = ['apple', 'banana', 'cherry']
print(fruits[1])  # Output: banana

Dictionary: Dictionaries are indexed by keys, which can be of various immutable data types (e.g., strings, numbers, tuples). You access elements using these keys.
Example:
person = {'name': 'Alice', 'age': 25, 'city': 'New York'}
print(person['age'])  # Output: 25

Order

List: Lists maintain the order of elements. The order in which items are added is preserved.
Example:
numbers = [1, 2, 3, 4]
print(numbers)  # Output: [1, 2, 3, 4]

Dictionary: Dictionaries do not guarantee any specific order of elements. As of Python 3.7+, dictionaries preserve insertion order, but this was not guaranteed in earlier versions.
Example:
data = {'first': 'Alice', 'second': 'Bob'}
print(data)  # Output (in Python 3.7+): {'first': 'Alice', 'second': 'Bob'}

Use Cases

List: Lists are used when you need to store an ordered collection of items. They are ideal for sequences of elements that you want to iterate over or modify by index.
Example Use Cases:

A list of student names.
A collection of numbers for statistical analysis.

Dictionary: Dictionaries are used when you need to store data in key-value pairs. They are ideal for situations where you want to quickly look up a value based on a unique key.
Example Use Cases:

A dictionary of employee records with employee IDs as keys.
A mapping of state abbreviations to state names.

Performance

List: Accessing elements in a list by index is very fast (O(1) time complexity), but searching for an element by value is slower (O(n) time complexity).
Dictionary: Dictionaries offer fast lookups, insertions, and deletions based on keys (average O(1) time complexity). This is due to the underlying hash table implementation.

Methods and Operations

List: Lists come with methods like append(), extend(), remove(), pop(), and sort().
Example:
fruits = ['apple', 'banana']
fruits.append('cherry')
print(fruits)  # Output: ['apple', 'banana', 'cherry']

Dictionary: Dictionaries provide methods such as keys(), values(), items(), get(), pop(), and update().
Example:
person = {'name': 'Alice', 'age': 25}
person.update({'city': 'New York'})
print(person)  # Output: {'name': 'Alice', 'age': 25, 'city': 'New York'}

Practical Examples
Example 1: List
# Creating a list
numbers = [1, 2, 3, 4, 5]

# Accessing an element by index
print(numbers[2])  # Output: 3

# Adding an element
numbers.append(6)
print(numbers)  # Output: [1, 2, 3, 4, 5, 6]

Example 2: Dictionary
# Creating a dictionary
person = {'name': 'Alice', 'age': 25, 'city': 'New York'}

# Accessing a value by key
print(person['name'])  # Output: Alice

# Adding a key-value pair
person['profession'] = 'Engineer'
print(person)  # Output: {'name': 'Alice', 'age': 25, 'city': 'New York', 'profession': 'Engineer'}

Conclusion
Lists and dictionaries are fundamental data structures in Python, each with unique features and use cases. Lists are suitable for ordered collections of items, while dictionaries are ideal for associating values with unique keys. Understanding their differences helps in choosing the right data structure for your needs.

What is the difference between a list and a dictionary in Python?

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