Python List

Syntax

A Python list is a collection of items that are ordered and changeable. Lists are written with square brackets, and the items are separated by commas.

Here is an example of a list in Python:

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

You can also create a list of strings or a list of mixed data types:

my_list = ['a', 'b', 'c', 'd']
my_list = [1, 'a', True, 3.14]

You can access the items in a list by referring to their index number. The index numbers start at 0 for the first item, 1 for the second item, and so on.

first_item = my_list[0]
second_item = my_list[1]

You can also use negative index numbers to access items from the end of the list. For example, -1 refers to the last item, -2 refers to the second last item, and so on.

last_item = my_list[-1]
second_last_item = my_list[-2]

List characteristics

Here are some characteristics of Python lists:

Lists are ordered, meaning that the items are stored in a specific order, and that order is preserved. To demonstrate this, we can create a list and access the items using their index numbers:

my_list = ['a', 'b', 'c', 'd']

print(my_list[0])  # 'a'
print(my_list[1])  # 'b'
print(my_list[2])  # 'c'
print(my_list[3])  # 'd'

Lists are changeable, meaning that you can add, remove, or modify the items in a list. To demonstrate this, we can use the append() method to add an item to the end of a list, the insert() method to add an item at a specific index, and the remove() method to remove an item from a list:

my_list = ['a', 'b', 'c', 'd']

# Add an item to the end of the list
my_list.append('e')
print(my_list)  # ['a', 'b', 'c', 'd', 'e']

# Add an item at a specific index
my_list.insert(1, 'f')
print(my_list)  # ['a', 'f', 'b', 'c', 'd', 'e']

# Remove an item from the list
my_list.remove('b')
print(my_list)  # ['a', 'f', 'c', 'd', 'e']

You can also modify an item in a list by accessing the item using its index and assigning it a new value:

my_list = ['a', 'b', 'c', 'd']

my_list[1] = 'f'
print(my_list)  # ['a', 'f', 'c', 'd']

Lists can contain any data type, including integers, strings, and even other lists. To demonstrate this, we can create a list of mixed data types:

my_list = [1, 'a', True, 3.14, [1, 2, 3]]

print(my_list[0])  # 1
print(my_list[1])  # 'a'
print(my_list[2])  # True
print(my_list[3])  # 3.14
print(my_list[4])  # [1, 2, 3]

List Use Cases

Storing a collection of items

One common use case for lists is to store a collection of items. For example, you might have a list of student names, a list of numbers, or a list of strings.

student_names = ['Alice', 'Bob', 'Charlie', 'David']
numbers = [1, 2, 3, 4, 5]
words = ['apple', 'banana', 'cherry']

Iterating over a collection of items

Another common use case for lists is to iterate over the items in a list using a for loop. This is useful when you want to perform the same action on each item in the list.

student_names = ['Alice', 'Bob', 'Charlie', 'David']

for name in student_names:
    print(f'Hello, {name}!')

Hello, Alice!
Hello, Bob!
Hello, Charlie!
Hello, David!

Sorting a collection of items

You can use the built-in sorted() function to sort a list of items. This is useful when you want to order a list of items in a specific way.

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

sorted_numbers = sorted(numbers)
print(sorted_numbers)  # [1, 2, 3, 4, 5]

# You can also reverse the order by passing the "reverse" parameter
reverse_sorted_numbers = sorted(numbers, reverse=True)
print(reverse_sorted_numbers)  # [5, 4, 3, 2, 1]

Checking for membership

You can use the in operator to check if an item is in a list. This is useful when you want to check if a value is present in a list.

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

if 3 in numbers:
    print('3 is in the list')
else:
    print('3 is not in the list')

3 is in the list

Best Practices

Note that some of the following points include concepts which will be covered in later chapters.

Use the built-in len() function to get the length of a list. This is often more efficient than manually counting the number of elements in the list.

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

# Good
list_length = len(my_list)
print(list_length)  # 5

# Bad
list_length = 0
for item in my_list:
list_length += 1
print(list_length)  # 5

Use the in operator to check if an element is in a list. This is often more efficient than manually searching for the element in the list.

my_list = ['a', 'b', 'c', 'd']

# Good
if 'c' in my_list:
print("'c' is in the list")

# Bad
for item in my_list:
if item == 'c':
print("'c' is in the list")
break

Use the built-in sorted() function to sort a list. This is often more efficient than manually sorting the list.

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

# Good
sorted_list = sorted(my_list)
print(sorted_list)  # [1, 2, 3, 4, 5]

# Bad
for i in range(len(my_list)):
for j in range(i+1, len(my_list)):
if my_list[i] > my_list[j]:
my_list[i], my_list[j] = my_list[j], my_list[i]
print(my_list)  # [1, 2, 3, 4, 5]

Use list comprehension to create a new list based on an existing list. This is often more concise and efficient than using a loop.

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

# Good
new_list = [item * 2 for item in my_list]
print(new_list)  # [2, 4, 6, 8, 10]

# Bad
new_list = []
for item in my_list:
new_list.append(item * 2)
print(new_list)  # [2, 4, 6, 8, 10]

Use the enumerate() function to iterate over a list and get the index and value of each element. This is often more convenient than using a loop with the range() function.

my_list = ['a', 'b', 'c', 'd']

# Good
for i, item in enumerate(my_list):
print(f"Index {i}: {item}")

# Bad
for i in range(len(my_list)):
print(f"Index {i}: {my_list[i]}")