Python List Basics

Introduction

In Python, a list is a type of data structure that can store a collection of items. Lists are a fundamental part of a programmer's repertoire, and are often used whenever many values need to be kept track of at once. In this blog, we will cover the basics of using lists in Python.

Initializing a List

First, let's create a list to work with. Lists can be declared by separating elements with commas, and enclosing them in []. Elements in a list do not have to be all of the same type, even if these demonstrations are only using strings.

#example of initializing a list in Python with square brackets
my_list = ["This", "is", "a", "list"]
print(my_list)

Output:

["This", "is", "a", "list"]

When setting these starting values, you can also use variables as values.

#example of initializing a list in Python with variables
a = "These"
b = "are"
c = "some"
d = "variables"
my_list = [a, b, c, d]
print(my_list)

Output:

["These", "are", "some", "variables"]

Declaring with square brackets allows you to set initial values for your list, although you can initialize an empty list this way as well.

#empty list in python
my_list = []
print(my_list)

Output:

[]

Accessing Elements in a List

Elements in a list are indexed. Each element has a number that corresponds to their position in the list. The first element will have index 0, the second element 1, the third 2, and so on. To access an element in a list by index, simply place [n] after the name of your list, with n being the index of the element you wish to get.

#example of accessing an element by index
my_list = ["This", "is", "a", "list"]
print(my_list[3])

Output:

list

Using a negative index will select elements from the back of the list.

#example of using negative indexes
my_list = ["This", "is", "a", "list"]
print(my_list[-3])

Output:

is

You can also access several values from a list at once by splicing it. To splice a list, use the following syntax:

my_list[start_index:end_index]

Doing so gives a new list with every element in my_list from index start_index to end_index. This new list includes the element at start_index, and excludes the element at end_index.

#example of splicing a list
my_list = ["This", "is", "a", "list"]
print(my_list[1:3])

Output:

["is", "a"]

Leaving start_index blank will splice from the start of the list (every element before end_index). Likewise, leaving end_index blank will splice to the back of the list (every element at and after start_index). Leaving both blank will create a complete copy of the list.

#example of more splicing
my_list = ["This", "is", "a", "list"]
print(my_list[1:])
print(my_list[:3])
print(my_list[:])

Output:

["is", "a", "list"] ["This", "is", "a"] ["This", "is", "a", "list"]

When working with indexes, always keep in mind the size of your lists. Using an index that is not in the list will create an error and stop your code.

#example of bad list indexing
my_list = ["This", "is", "a", "list"]
print(my_list[4])

Output:

IndexError: list index out of range

To help with this, we can use the built-in function len() to get the length of the list, with the following syntax:

len(list_name)

where list_name is your list.

#example of using the len() function
my_list = ["This", "is", "a", "list"]
print(len(my_list))

Output:

4

Changing the Contents of a List

To change the element at an index in a list, we can use the assignment operator =.

#example of changing an element in a list
my_list = ["This", "is", "a", "list"]
my_list[2] = "another"
print(my_list)

Output:

["This", "is", "another", "list"]

However, modifying lists in ways that change their size will require the use of methods. For example, we can add elements to the end of a list using the built-in .append() method. Its syntax is as follows:

list.append(value)

where value is the element you want to add.

#example of appending an element to a list
my_list = ["This", "is", "a", "list"]
my_list.append("too")
print(my_list)

Output:

["This", "is", "a", "list", "too"]

You can also add elements into the middle of the list using the .insert() method. Its syntax is as follows:

list.insert(index, value)

where list is the name of the list, and index is the position you want to add value at. All elements at and after index will have their index increased by one to make space for the new element.

#example of inserting an element into a list
my_list = ["This", "is", "a", "list"]
my_list.append(2, "also")
print(my_list)

Output:

["This", "is", "also", "a", "list"]

Finally, we can also delete elements from lists using the del keyword. Simply place the element to be deleted after del, and it will be removed. Elements after it will be shifted up by one to fill the gap.

#example of deleting an element from a list
my_list = ["This", "is", "a", "list"]
del my_list[1]
print(my_list)

Output:

["This", "a", "list"]

Looping Through Lists

Sometimes, we want to be able to interact with elements in a list individually. When lists can contain hundreds, thousands, maybe even millions of elements, there is no way we can write out code for each individual value. Instead, we can use loops to greatly shorten our code. The simplest and most common way is to use a for loop to iterate through our list. When we use a for loop directly on a list, it will execute the enclosed code with the iterating variable set to elements of the list until the list has been exhausted. The values will be taken from the list in increasing index order, starting with index 0.

#example of using a for loop to iterate through a list
my_list = ["This", "is", "a", "list"]
for value in my_list:
    print(value)

Output:

This

is

a

list

However, there are some situations where you will also need the corresponding indexes to your values. In this case, we can use a for i in range() loop. Combining this with the len() function, we can now iterate through every index in the list, which we can then use to access elements.

#example of using a for i in range() loop to iterate through a list
my_list = ["This", "is", "a", "list"]
for i in range(len(my_list)):
    print("Element at index " + str(i) + " : " + my_list[i])

Output:

Element at index 0 : This

Element at index 1 : is

Element at index 2 : a

Element at index 3 : list

Conclusion

Lists in Python are versatile and powerful data structures that can be found in almost any program. There are even more ways to use lists (which we will cover in a different blog post), but for now, you are another step closer to mastering Python! Happy Coding!