A list in Python is a versatile and fundamental data structure used to store an ordered collection of items. It is one of the most commonly used data structures for grouping data. Lists are created by placing items inside square brackets [], separated by commas.

Key Characteristics of Lists

• Ordered: The items in a list have a defined order, and that order will not change unless you modify it.

• Mutable: This is a crucial feature. "Mutable" means that you can change the list after it has been created. You can add, remove, or modify its elements directly.

• Allows Duplicates: Lists can contain multiple items with the same value.

• Can Contain Different Data Types: A single list can hold items of different data types, such as integers, strings, and even other lists.

Python

# A list of fruits (all strings)

fruits = ["apple", "banana", "cherry"]

# A list with mixed data types, including a nested list

mixed_list = [1, "hello", 3.14, True, ["another", "list"]]

print(f"A simple list: {fruits}")

print(f"A mixed-type list: {mixed_list}")

Operations with Operators

Concatenation (+)

Combines two lists to create a new, single list.

Python

list1 = [1, 2, 3]

list2 = [4, 5, 6]

combined_list = list1 + list2

print(f"Concatenation (+): {combined_list}")

# Output: [1, 2, 3, 4, 5, 6]

Repetition (*)

Creates a new list by repeating the original list's elements a specified number of times.

Python

repeated_list = ['a'] * 5

print(f"Repetition (*): {repeated_list}")

# Output: ['a', 'a', 'a', 'a', 'a']

Indexing ([])

Accesses a single item at a specific position (index). Python uses zero-based indexing, so the first item is at index 0. Negative indexing starts from the end, where -1 is the last item.

Python

numbers = [10, 20, 30, 40, 50]

print(f"First item (index 0): {numbers[0]}")      # Output: 10

print(f"Last item (index -1): {numbers[-1]}")     # Output: 50

Slicing ([:])

Extracts a portion of the list, creating a new list. The syntax is list[start:stop:step]. The start index is inclusive, and the stop index is exclusive.

Python

numbers = [10, 20, 30, 40, 50, 60]

print(f"Slice from index 2 to 4: {numbers[2:5]}") # Output: [30, 40, 50]

print(f"Slice every second element: {numbers[::2]}") # Output: [10, 30, 50]

Membership Testing (in, not in)

Checks if an item exists within a list, returning True or False.

Python

fruits = ["apple", "banana", "cherry"]

print(f"Is 'banana' in the list? {'banana' in fruits}")       # Output: True

print(f"Is 'mango' not in the list? {'mango' not in fruits}") # Output: True

Built-in Functions for Lists

len()

Returns the number of items in a list.

Python

numbers = [10, 20, 30, 40]

print(f"Length of the list: {len(numbers)}") # Output: 4

sum(), min(), max()

Returns the sum, minimum, or maximum item in a list. These only work if all items are numbers.

Python

numbers = [5, 2, 8, 1, 9]

print(f"Sum of numbers: {sum(numbers)}")   # Output: 25

print(f"Min of numbers: {min(numbers)}")   # Output: 1

print(f"Max of numbers: {max(numbers)}")   # Output: 9

sorted()

Returns a new, sorted list without modifying the original list.

Python

unsorted_numbers = [3, 1, 4, 1, 5, 9]

sorted_copy = sorted(unsorted_numbers)

print(f"Original list: {unsorted_numbers}")

print(f"New sorted list: {sorted_copy}")

Common List Methods

Adding and Removing Items

• .append(item): Adds a single item to the very end of the list.

Python

fruits = ["apple", "banana"]

fruits.append("cherry")

print(f"After .append('cherry'): {fruits}") # Output: ['apple', 'banana', 'cherry']

• .extend(iterable): Adds all the items from an iterable (like another list) to the end of the list.

Python

fruits = ["apple", "banana"]

fruits.extend(["cherry", "mango"])

print(f"After .extend(): {fruits}") # Output: ['apple', 'banana', 'cherry', 'mango']

• .insert(index, item): Inserts an item at a specified position.

Python

fruits = ["apple", "cherry"]

fruits.insert(1, "banana")

print(f"After .insert(1, 'banana'): {fruits}") # Output: ['apple', 'banana', 'cherry']

• .remove(item): Removes the first occurrence of a specified item.

Python

fruits = ["apple", "banana", "cherry", "banana"]

fruits.remove("banana")

print(f"After .remove('banana'): {fruits}") # Output: ['apple', 'cherry', 'banana']

• .pop(index): Removes and returns the item at a specified index. If no index is given, it removes and returns the last item.

Python

fruits = ["apple", "banana", "cherry"]

removed_fruit = fruits.pop(1)

print(f"Removed fruit: {removed_fruit}, List is now: {fruits}") # Output: Removed fruit: banana, List is now: ['apple', 'cherry']

• .clear(): Removes all items from the list.

Python

fruits = ["apple", "banana", "cherry"]

fruits.clear()

print(f"After .clear(): {fruits}") # Output: []

Organizing and Searching

• .sort(): Sorts the items of the list in place (modifies the original list).

Python

numbers = [5, 2, 8, 1, 9]

numbers.sort()

print(f"After .sort(): {numbers}") # Output: [1, 2, 5, 8, 9]

• .reverse(): Reverses the order of the items in place.

Python

numbers = [1, 2, 5, 8, 9]

numbers.reverse()

print(f"After .reverse(): {numbers}") # Output: [9, 8, 5, 2, 1]

• .index(item): Returns the index of the first occurrence of a specified item.

Python

fruits = ["apple", "banana", "cherry"]

print(f"Index of 'cherry': {fruits.index('cherry')}") # Output: 2

• .count(item): Returns the number of times a specified item appears in the list.

Python

numbers = [1, 2, 8, 2, 5, 2]

print(f"Count of '2': {numbers.count(2)}") # Output: 3

Copying

• .copy(): Returns a shallow copy of the list. This is important when you want to modify a copy without affecting the original list.

Python

original_list = [1, 2, 3]

copied_list = original_list.copy()

copied_list.append(4)

print(f"Original list: {original_list}") # Output: [1, 2, 3]

print(f"Copied list: {copied_list}")   # Output: [1, 2, 3, 4]