📁 Lists¶

1. Definition and Initialization¶

In Python, a list is an ordered collection of items, which can be of any type and are enclosed in square brackets.

In [1]:

# list of integers
list1 = [1, 2, 3, 4, 5]
print(list1)  # prints: [1, 2, 3, 4, 5]

# list of strings
list2 = ['apple', 'banana', 'cherry']
print(list2)  # prints: ['apple', 'banana', 'cherry']

# mixed list
list3 = [1, 'apple', True, 3.14, [1, 2, 3]]
print(list3)  # prints: [1, 'apple', True, 3.14, [1, 2, 3]]

# list of integers list1 = [1, 2, 3, 4, 5] print(list1) # prints: [1, 2, 3, 4, 5] # list of strings list2 = ['apple', 'banana', 'cherry'] print(list2) # prints: ['apple', 'banana', 'cherry'] # mixed list list3 = [1, 'apple', True, 3.14, [1, 2, 3]] print(list3) # prints: [1, 'apple', True, 3.14, [1, 2, 3]]

[1, 2, 3, 4, 5]
['apple', 'banana', 'cherry']
[1, 'apple', True, 3.14, [1, 2, 3]]

2. Accessing List Items¶

You can access list items by referring to their index number. Python uses 0-based indexing.

In [2]:

list1 = [1, 2, 3, 4, 5]
print(list1[0])  # prints: 1
print(list1[2])  # prints: 3

# Negative indexing starts from the end.
print(list1[-1])  # prints: 5

list1 = [1, 2, 3, 4, 5] print(list1[0]) # prints: 1 print(list1[2]) # prints: 3 # Negative indexing starts from the end. print(list1[-1]) # prints: 5

1
3
5

3. Modifying List Items¶

You can modify list items by referring to their index number.

In [3]:

list1 = [1, 2, 3, 4, 5]
list1[0] = 10
print(list1)  # prints: [10, 2, 3, 4, 5]

list1 = [1, 2, 3, 4, 5] list1[0] = 10 print(list1) # prints: [10, 2, 3, 4, 5]

[10, 2, 3, 4, 5]

4. List Slicing¶

You can extract a part of a list using slicing. The syntax is list[start:stop:step].

5. List Methods¶

Python provides several methods to manipulate lists, like append(), insert(), remove(), pop(), sort(), and many more.

In [4]:

list1 = [1, 2, 3, 4, 5]

# add an item to the end of the list
list1.append(6)
print(list1)  # prints: [1, 2, 3, 4, 5, 6]

# add an item at a specified position
list1.insert(0, 0)
print(list1)  # prints: [0, 1, 2, 3, 4, 5, 6]

# remove a specified item
list1.remove(0)
print(list1)  # prints: [1, 2, 3, 4, 5, 6]

# remove an item at a specified position
list1.pop(0)
print(list1)  # prints: [2, 3, 4, 5, 6]

# sort the list
list1.sort(reverse=True)
print(list1)  # prints: [6, 5, 4, 3, 2]

list1 = [1, 2, 3, 4, 5] # add an item to the end of the list list1.append(6) print(list1) # prints: [1, 2, 3, 4, 5, 6] # add an item at a specified position list1.insert(0, 0) print(list1) # prints: [0, 1, 2, 3, 4, 5, 6] # remove a specified item list1.remove(0) print(list1) # prints: [1, 2, 3, 4, 5, 6] # remove an item at a specified position list1.pop(0) print(list1) # prints: [2, 3, 4, 5, 6] # sort the list list1.sort(reverse=True) print(list1) # prints: [6, 5, 4, 3, 2]

[1, 2, 3, 4, 5, 6]
[0, 1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6]
[2, 3, 4, 5, 6]
[6, 5, 4, 3, 2]

6. List Comprehensions¶

List comprehensions provide a concise way to create lists. It consists of brackets containing an expression followed by a for statement, optionally followed by for or if clauses.

In [5]:

# create a list of squares for numbers from 0 to 9
squares = [x**2 for x in range(10)]
print(squares)  # prints: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# create a list of even numbers from the above list
evens = [x for x in squares if x % 2 == 0]
print(evens)  # prints: [0, 4, 16, 36, 64]

# create a list of squares for numbers from 0 to 9 squares = [x**2 for x in range(10)] print(squares) # prints: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] # create a list of even numbers from the above list evens = [x for x in squares if x % 2 == 0] print(evens) # prints: [0, 4, 16, 36, 64]

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
[0, 4, 16, 36, 64]

7. Nested Lists¶

A list can contain other lists. This is called a nested list.

In [6]:

nested = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(nested)  # prints: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
print(nested[0])  # prints: [1, 2, 3]
print(nested[0][1])  # prints: 2

nested = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] print(nested) # prints: [[1, 2, 3], [4, 5, 6], [7, 8, 9]] print(nested[0]) # prints: [1, 2, 3] print(nested[0][1]) # prints: 2

[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
[1, 2, 3]
2

8. Length of a List¶

You can get the number of items in a list with the len() function.

In [7]:

list1 = [1, 2, 3, 4, 5]
print(len(list1))  # prints: 5

list1 = [1, 2, 3, 4, 5] print(len(list1)) # prints: 5

5