List

In your programs, you often need to group several elements in order to process them as a single object. For this, you will need to use different collections. One of the most important and useful collections in Python is a list.

Creating and printing lists

Look at a simple list that stores several names of dogs' breeds:

dog_breeds = ['corgi', 'labrador', 'poodle', 'jack russell']
print(dog_breeds)  # ['corgi', 'labrador', 'poodle', 'jack russell']

In the first line, we use square brackets to create a list that contains four elements and then assign it to the dog_breeds variable. In the second line, the list is printed through the variable's name. All the elements are printed in the same order as they were stored in the list because lists are ordered.

Here is another list that contains five integers:

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

Another way to create a list is to invoke the list function. It is used to create a list out of an iterable object: that is, a kind of object where you can get its elements one by one. The concept of iterability will be explained in detail further on, but let's look at the examples below:

list_out_of_string = list('danger!')
print(list_out_of_string)  # ['d', 'a', 'n', 'g', 'e', 'r', '!']

list_out_of_integer = list(235)  # TypeError: 'int' object is not iterable

So, the list function creates a list containing each element from the given iterable object. For now, remember that a string is an example of an iterable object, and an integer is an example of a non-iterable object. A list itself is also an iterable object.

Let's also note the difference between the list function and creating a list using square brackets:

multi_element_list = list('danger!')
print(multi_element_list)  # ['d', 'a', 'n', 'g', 'e', 'r', '!']

single_element_list = ['danger!']
print(single_element_list)  # ['danger!']

The square brackets and the list function can also be used to create empty lists that do not have elements at all.

empty_list_1 = list()
empty_list_2 = []

In the following topics, we will consider how to fill empty lists.

Features of lists

Lists can store duplicate values as many times as needed.

on_off_list = ['on', 'off', 'on', 'off', 'on']
print(on_off_list)  # ['on', 'off', 'on', 'off', 'on']

Another important thing about lists is that they can contain different types of elements, including other lists. So there are neither restrictions nor fixed list types, and you can add any data you want to your list, as in the following example:

different_objects = ['a', 1, 'b', 2, [1, 2, 3]]

Length of a list

Sometimes you need to know how many elements there are in a list. There is a built-in function called len() that can be applied to any iterable object, and it simply returns the length of that object.

So, when applied to a list, it returns the number of elements in that list:

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

empty_list = list()
print(len(empty_list))  # 0

single_element_list = ['danger!']
print(len(single_element_list))  # 1

multi_elements_list = list('danger!')
print(len(multi_elements_list))  # 7

In the example above, you can see how the len() function works. Again, pay attention to the difference between list() and [] as applied to strings: it may not result in what you expected.

Summary

As a recap, we note that lists are:

• ordered, i.e. each element has a fixed position in a list;

• iterable, i.e. you can get their elements one by one;

• able to store duplicate values;

• able to store different types of elements;

• besides being used for creating an empty list, the list() function also can be used to make a list out of an iterable object.

Read more on this topic in Python range function and its methods on Hyperskill Blog.