What is a Python List?
A Python list is a collection of items enclosed within square brackets ([ ]) and separated by commas. Each item in the list is called an element. Lists can hold any type of data, including numbers, strings, or even other lists.
Syntax:
list_name = [element1, element2, element3, ...]
Example:
# List of integers
numbers = [1, 2, 3, 4, 5]
# List of strings
fruits = ["apple", "banana", "cherry"]
# Mixed data types
mixed_list = [1, "hello", 3.14, True]
# Nested list
nested_list = [[1, 2], [3, 4], [5, 6]]
Characteristics of Lists
- Ordered:
Elements are stored in a specific sequence, and this order is maintained. - Mutable:
Lists can be updated or modified after creation. - Allow Duplicates:
Lists can store duplicate values. - Dynamic:
The size of a list can grow or shrink as needed.
Accessing List Elements
You can access elements in a list using their index. Indexing in Python starts from 0.
Syntax:
list_name[index]
Example:
fruits = ["apple", "banana", "cherry"]
print(fruits[0]) # Output: apple
print(fruits[1]) # Output: banana
print(fruits[-1]) # Output: cherry (negative index starts from the end)
Slicing Lists
Slicing allows you to extract a subset of the list.
Syntax:
list_name[start:end:step]
- start: Starting index (inclusive).
- end: Ending index (exclusive).
- step: Step size (optional).
Example:
numbers = [0, 1, 2, 3, 4, 5, 6]
# Slice from index 1 to 4
print(numbers[1:5]) # Output: [1, 2, 3, 4]
# Slice with step
print(numbers[0:6:2]) # Output: [0, 2, 4]
# Slice with negative step (reverse)
print(numbers[::-1]) # Output: [6, 5, 4, 3, 2, 1, 0]
Common List Operations
1. Adding Elements
- append(): Adds an element to the end of the list.
- insert(): Inserts an element at a specific index.
- extend(): Adds multiple elements to the list.
fruits = ["apple", "banana"]
fruits.append("cherry")
print(fruits) # Output: ['apple', 'banana', 'cherry']
fruits.insert(1, "orange")
print(fruits) # Output: ['apple', 'orange', 'banana', 'cherry']
fruits.extend(["grape", "melon"])
print(fruits) # Output: ['apple', 'orange', 'banana', 'cherry', 'grape', 'melon']
2. Removing Elements
- remove(): Removes the first occurrence of a value.
- pop(): Removes an element by index (default: last).
- clear(): Removes all elements.
fruits = ["apple", "banana", "cherry"]
fruits.remove("banana")
print(fruits) # Output: ['apple', 'cherry']
fruits.pop(0)
print(fruits) # Output: ['cherry']
fruits.clear()
print(fruits) # Output: []
List Methods
| Method | Description |
|---|---|
| append() | Adds an element to the end of the list. |
| insert() | Inserts an element at a specified position. |
| extend() | Adds multiple elements to the end of the list. |
| remove() | Removes the first occurrence of a specified element. |
| pop() | Removes an element by index or the last element. |
| clear() | Clears all elements from the list. |
| index() | Returns the index of the first occurrence of a value. |
| count() | Counts the occurrences of a specified element. |
| sort() | Sorts the list in ascending or descending order. |
| reverse() | Reverses the order of the list. |
| copy() | Returns a shallow copy of the list. |
Iterating Through a List
You can use a for loop to iterate through the elements of a list.
Example:
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
List Comprehensions
List comprehensions provide a concise way to create lists.
Syntax:
new_list = [expression for item in iterable if condition]
Example:
# Create a list of squares
squares = [x**2 for x in range(5)]
print(squares) # Output: [0, 1, 4, 9, 16]
# Filter even numbers
even_numbers = [x for x in range(10) if x % 2 == 0]
print(even_numbers) # Output: [0, 2, 4, 6, 8]
Nested Lists
Lists can contain other lists, enabling the creation of multi-dimensional data structures.
Example:
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
# Accessing elements
print(matrix[0]) # Output: [1, 2, 3]
print(matrix[1][2]) # Output: 6