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Python 101: Interactively learn how to program with Python 3

Lists

Explore the fundamentals of Python lists including their creation with different data types, combining lists, sorting techniques, and slicing. Understand in-place sorting behavior to manage list data effectively in your programming projects.

We'll cover the following...

Creating a list

A Python list is similar to an array in other languages. In Python, an empty list can be created in the following ways.

my_list = []
my_list = list()

As you can see, you can create the list using square brackets or by using the Python built-in, list. A list contains a list of elements, such as strings, integers, objects or a mixture of types. Let’s take a look at some examples:

my_list = [1, 2, 3]
my_list2 = ["a", "b", "c"]
my_list3 = ["a", 1, "Python", 5]

The first list has 3 integers, the second has 3 strings and the third has a mixture. You can also create lists of lists like this:

Python 3.5
my_list = [1, 2, 3]
my_list2 = ["a", "b", "c"]
my_nested_list = [my_list, my_list2]
print(my_nested_list) # [[1, 2, 3], ['a', 'b', 'c']]

Combining two lists

Occasionally, you’ll want to combine two lists together. The first way is to use the extend method:

Python 3.5
combo_list = []
one_list = [4, 5]
combo_list.extend(one_list)
print(combo_list) # [4, 5]

A slightly easier way is to just add two lists together (yes, it really is that easy).

Python 3.5
my_list = [1, 2, 3]
my_list2 = ["a", "b", "c"]
combo_list = my_list + my_list2
print(combo_list) # [1, 2, 3, 'a', 'b', 'c']

Sort a list

You can also sort a list. Let’s spend a moment to see how to do that:

Python 3.5
alpha_list = [34, 23, 67, 100, 88, 2]
alpha_list.sort()
print(alpha_list) # [2, 23, 34, 67, 88, 100]

Now there is a got-cha above. Can you see it? Let’s do one more example to make it obvious:

Python 3.5
alpha_list = [34, 23, 67, 100, 88, 2]
sorted_list = alpha_list.sort()
print(sorted_list)

In this example, we try to assign the sorted list to a variable. However, when you call the sort() method on a list, it sorts the list in-place. So if you try to assign the result to another variable, then you’ll find out that you’ll get a None object, which is like a Null in other languages. Thus when you want to sort something, just remember that you sort them in-place and you cannot assign it to a different variable.

Slice a list

You can slice a list just like you do with a string:

Python 3.5
alpha_list = [34, 23, 67, 100, 88, 2]
alpha_list.sort()
print(alpha_list[0:3])
# [2, 23, 34]

This code returns a list of just the first 3 elements.