Introducing Lists

 

    In this blog and the next you’ll learn what lists are and how to start working with the elements in a list. Lists allow you to store sets of information in one place, whether you have just a few items or millions of items. Lists are one of Python’s most powerful features readily accessible to new programmers, and they tie together many important concepts in programming.

What is a List?

A list is a data structure in Python that is a mutable, or changeable, ordered sequence of elements. Each element or value that is inside of a list is called an item. Just as strings are defined as characters between quotes, lists are defined by having values between square brackets [ ] and individual elements in the list are separated by commas.

Here’s a simple example of a list that contains a few kinds of bicycles:

bicycles = ['trek', 'cannondale', 'redline', 'specialized']

print(bicycles)     gives  ['trek', 'cannondale', 'redline', 'specialized']

Accessing Elements in a List

    In order to access an element in a list, write the name of the list followed by the index of the item enclosed in square brackets.

Note: Python considers the first item in a list to be at position 0, not position 1.

print(bicycles[0]) gives trek.

print(bicycles[2]) gives redline.

Using Individual Values from a List

message = "My first bicycle was a " + bicycles[0].title() + "."

print(message) gives My first bicycle was a Trek.

Changing, Adding, and Removing Elements

Most lists you create will be dynamic, meaning you’ll build a list and then add and remove elements from it as your program runs its course.

Modifying Elements in a List

The syntax for modifying an element is similar to the syntax for accessing an element in a list.

For example, let’s say we have a list of motorcycles, and the first item in the list is 'honda'. How would we change the value of this first item?

motorcycles = ['honda', 'yamaha', 'suzuki'] 

print(motorcycles)  gives ['honda', 'yamaha', 'suzuki']  

motorcycles[0] = 'ducati'

print(motorcycles) gives ['ducati', 'yamaha', 'suzuki']

Adding Elements to a List

Appending Elements to the End of a List

The simplest way to add a new element to a list is to append the item to the list. When you append an item to a list, the new element is added to the end of the list

motorcycles.append('ducati')

print(motorcycles) gives ['honda', 'yamaha', 'suzuki', 'ducati']

Inserting Elements into a List

You can add a new element at any position in your list by using the insert() method. You do this by specifying the index of the new element and the value of the new item.

motorcycles.insert(0, 'ducati')

print(motorcycles) gives ['ducati', 'honda', 'yamaha', 'suzuki']

Removing Elements from a List

Removing an Item Using the del Statement

If you know the position of the item you want to remove from a list, you can use the del statement. 

motorcycles = ['honda', 'yamaha', 'suzuki']

del motorcycles[0] 

print(motorcycles) gives ['yamaha', 'suzuki']

Removing an Item Using the pop() Method

Sometimes you’ll want to use the value of an item after you remove it from a list. The pop() method removes the last item in a list, but it lets you work with that item after removing it.

popped_motorcycle = motorcycles.pop()  

print(motorcycles)  gives ['honda', 'yamaha']

print(popped_motorcycle) gives suzuki.

You can actually use pop() to remove an item in a list at any position by including the index of the item you want to remove in parentheses.

popped_motorcycle = motorcycles.pop(0)

Removing an Item by Value

Sometimes you won’t know the position of the value you want to remove from a list. If you only know the value of the item you want to remove, you can use the remove() method.

motorcycles = ['honda', 'yamaha', 'suzuki', 'ducati'] 

motorcycles.remove('ducati') 

print(motorcycles) gives ['honda', 'yamaha', 'suzuki'] 

  

Organizing a List

Sorting a List Permanently with the sort() Method

cars = ['bmw', 'audi', 'toyota', 'subaru']

cars.sort()

print(cars) gives ['audi', 'bmw', 'subaru', 'toyota'] 

You can also sort this list in reverse alphabetical order by passing the argument reverse=True to the sort() method. 

cars.sort(reverse=True) 

print(cars) gives ['toyota', 'subaru', 'bmw', 'audi']

Sorting a List Temporarily with the sorted() Function

To maintain the original order of a list but present it in a sorted order, you can use the sorted() function. The sorted() function lets you display your list in a particular order but doesn’t affect the actual order of the list.

print(sorted(cars)) gives ['audi', 'bmw', 'subaru', 'toyota'] 

print(cars) gives ['bmw', 'audi', 'toyota', 'subaru']

Printing a List in Reverse Order

cars.reverse()  

print(cars) gives ['subaru', 'toyota', 'audi', 'bmw']

The reverse() method changes the order of a list permanently, but you can revert to the original order anytime by applying reverse() to the same list a second time.

Finding the Length of a List

You can quickly find the length of a list by using the len() function

cars = ['bmw', 'audi', 'toyota', 'subaru'] 

print(len(cars)) gives 4.

Note: 

Keep in mind that whenever you want to access the last item in a list you use the index -1. This will always work, even if your list has changed size since the last time you accessed it.

motorcycles = ['honda', 'yamaha', 'suzuki'] 

print(motorcycles[-1]) gives suzuki.

Conclusion

In this blog you learned what lists are and how to work with the individual items in a list. You learned how to define a list and how to add and remove elements. You learned to sort lists permanently and temporarily for display purposes. 

In next blog you’ll learn how to work with items in a list more efficiently. By looping through each item in a list using just a few lines of code you’ll be able to work efficiently, even when your list contains thousands or millions of items.