Introducing lists
Python has many built-in types, such as strings and integers. Python has a type for storing a collection of values: the list.
Create a list
You create a list by assigning a sequence of values to a variable. Each value is separated by a comma and surrounded by brackets ([]). The following example stores the list of all planets in the planets variable:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
Access list items by index
You can access any item in a list by putting the index in brackets [] after the list variable's name. Indexes start from 0, so in the following code, planets[0] is the first item in the list planets:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
print("The first planet is", planets[0])
print("The second planet is", planets[1])
print("The third planet is", planets[2])
The first planet is Mercury
The second planet is Venus
The third planet is Earth
Note
Because all indexes start from 0, [1] is the second item, [2] is the third, and so forth.
You can also modify values in a list by using an index. You do so by assigning a new value, in much the same way that you would assign a variable value. For example, you could change the name of Mars in the list to use its nickname:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
planets[3] = "Red Planet"
print("Mars is also known as", planets[3])
Output: Mars is also known as Red Planet
Determine the length of a list
To get the length of a list, use the built-in function len(). The following code creates a new variable, number_of_planets. The code assigns that variable with the number of items in the list planets (8).
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
number_of_planets = len(planets)
print("There are", number_of_planets, "planets in the solar system.")
Output: There are 8 planets in the solar system
Add values to lists
Lists in Python are dynamic: you can add and remove items after they're created. To add an item to a list, use the method .append(value).
For example, the following code adds the string "Pluto" to the end of the list planets:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
planets.append("Pluto")
number_of_planets = len(planets)
print("There are actually", number_of_planets, "planets in the solar system.")
Output: There are actually 9 planets in the solar system.
Remove values from lists
You can remove the last item in a list by calling the .pop() method on the list variable:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune", "Pluto"]
planets.pop() # Goodbye, Pluto
number_of_planets = len(planets)
print("No, there are definitely", number_of_planets, "planets in the solar system.")
Use negative indexes
You saw how to use indexes to fetch an individual item in a list:
print("The first planet is", planets[0])
Output: The first planet is Mercury
Indexes start at zero and increase. Negative indexes start at the end of the list and work backward.
In the following example, an index of -1 returns the last item in a list. An index of -2 returns the second to last.
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
print("The last planet is", planets[-1])
print("The penultimate planet is", planets[-2])
The last planet is Neptune
The penultimate planet is Uranus
If you wanted to return the third to last, you'd use an index of -3 (and so on).
Find a value in a list
To determine where in a list a value is stored, you use the list's index method. This method searches for the value and returns the index of that item in the list. If it doesn't find a match, it returns -1.
The following example shows the use of "Jupiter" as the index value:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
jupiter_index = planets.index("Jupiter")
print("Jupiter is the", jupiter_index + 1, "planet from the sun")
Output: Jupiter is the 5 planet from the sun
Note
Because indexing starts with 0, you need to add 1 to display the proper number.
Here's another:
planets = ["Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune"]
mercury_index = planets.index("Mercury")
print("Mercury is the", mercury_index + 1, "planet from the sun")
Output: Mercury is the 1 planet from the sun