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

Question

Jayant Kumar · Accepted Answer

Table of Contents
Introduction to Lists and Dictionaries
In Python, lists and dictionaries are both versatile data structures used to store collections of data. However, they differ significantly in how they store and access data.
What is a List in Python?
A list is an ordered, mutable collection of items that can be of any data type. Lists are indexed by integers starting from 0. They allow duplicate values and maintain the order of items.
Example:
# Creating a list
fruits = ['apple', 'banana', 'cherry']

# Accessing items
print(fruits[0])  # Output: apple

# Modifying items
fruits[1] = 'blueberry'

# Adding items
fruits.append('orange')

# Removing items
fruits.remove('cherry')

print(fruits)  # Output: ['apple', 'blueberry', 'orange']

What is a Dictionary in Python?
A dictionary is an unordered, mutable collection of key-value pairs. Each key is unique and is used to access its corresponding value. Dictionaries do not maintain order prior to Python 3.7, but they do maintain insertion order starting from Python 3.7.
Example:
# Creating a dictionary
person = {'name': 'Alice', 'age': 25, 'city': 'New York'}

# Accessing values
print(person['name'])  # Output: Alice

# Modifying values
person['age'] = 26

# Adding new key-value pairs
person['profession'] = 'Engineer'

# Removing key-value pairs
del person['city']

print(person)  # Output: {'name': 'Alice', 'age': 26, 'profession': 'Engineer'}

Key Differences Between Lists and Dictionaries
1. Storage Structure:

List: Stores data in an ordered sequence, indexed by integers.
Dictionary: Stores data as key-value pairs, where each key maps to a value.

2. Indexing:

List: Uses integer indices to access elements. Example: fruits[0].
Dictionary: Uses unique keys to access values. Example: person['name'].

3. Order:

List: Maintains the order of elements. The order in which items are inserted is preserved.
Dictionary: Maintained order starting from Python 3.7. Earlier, it was unordered.

4. Mutability:

List: Mutable; you can change, add, or remove items.
Dictionary: Mutable; you can change, add, or remove key-value pairs.

5. Use Cases:

List: Best used when you need an ordered collection of items that can be indexed and may include duplicates. Example: A list of user names.
Dictionary: Best used when you need a collection of unique keys with associated values, allowing for fast lookups by key. Example: A dictionary mapping user IDs to user profiles.

Practical Examples
Example of Using a List:
# List of student names
students = ['Alice', 'Bob', 'Charlie']

# Accessing the first student's name
print(students[0])  # Output: Alice

Example of Using a Dictionary:
# Dictionary of student grades
grades = {'Alice': 'A', 'Bob': 'B', 'Charlie': 'C'}

# Accessing Alice's grade
print(grades['Alice'])  # Output: A

When to Use Each

Use a list when you need a collection of items in a specific order and when the position of each item matters.
Use a dictionary when you need to associate values with unique keys and perform quick lookups based on those keys.

Conclusion
Lists and dictionaries are fundamental data structures in Python, each with its own strengths and appropriate use cases. Lists are ideal for ordered collections, while dictionaries are perfect for mapping unique keys to values. Understanding the differences between these two structures will help you choose the right one for your data handling needs.

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

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