Working with Keys and Values

In the last section, we briefly mentioned how to access a value stored in a dictionary and how to update these values. In this section, we're going to dive a little deeper into how you can work with dictionary keys and values, and check for membership in a dictionary.

Dictionaries and loops

Let's start by listing the contents of a dictionary. We can create a new dictionary of students, where the keys are the students' names, and the values are their attendance percentages for a given course:

student_attendance = {
    "Rolf": 96,
    "Bob": 80,
    "Anne": 100
}

We can use the keys individually to get each attendance value, but when we're doing a repeated action like this, it's generally a good idea to use a loop. It makes our code shorter, easier to maintain, and easier to read.

Let's define a for loop to get each item in the dictionary:

for student in student_attendance:
    print(student)

Unfortunately, this doesn't quite do what we want. The code above will just print the keys for the dictionary, not the values.

There are a couple of ways we can get the output we want. The first is that we can use the list of keys to grab the values from the student_attendance dictionary like so:

for student in student_attendance:
    print(f"{student}: {student_attendance[student]}")

The other option is to use the .items() method. When iterating over the student_attendance dictionary, we can iterate over student_attendance.items() instead, which returns two items: the key, and the value. We can therefore provide two variables as part of our for loop to capture these values.

for student, attendance in student_attendance.items():
    print(f"{student}: {attendance}")

Either option would print:

Rolf: 96
Bob: 80
Anne: 100

tip

You can also iterate over a dictionaries values in a similar way using .values().

Checking for dictionary membership

We can easily check if a key exists in a particular dictionary using the in keyword, much like we do with lists.

Using the attendance example above, we might check if we have an attendance score for Bob like so:

if "Bob" in student_attendance:
    print(f"Bob: {student_attendance[student]}")
else:
    print("Bob isn't a student in this class!")