Storing lists in dictionaries#

You may recall that keys must be immutable. While this is true, there is no such requirement on values! We can use mutable objects like lists as values, and it is often very useful to do so.

A practical example is storing email addresses as a dictionary that relates domain names to user names. For example, suppose we have an email address, which is For this email address, notaperson is the username and is specific to the owner of the account. is the domain name and is shared by all people with email addresses. This organization lends itself well to dictionaries, especially when you consider that usernames don’t have to be unique! Just because there is doesn’t mean that doesn’t exist for example.

The reason this lends itself well to dictionaries is that there will likely be many usernames associated with each domain name.

Let’s create a dictionary using domain names as keys and a list of usernames as values.

emails = {"" : ["annie","joseph","savitha"],
"" : ["ben10","annie","dio"],
"" : ["joseph", "hotmail", "coda"]}

Notice that "annie" and "joseph" are usernames that are associated with different domain names. So, and both exist.

Let’s explore some things we can do with this dictionary through clever use of the tools we have. First, let’s try adding a username to the "" domain name. To add the username "yeohee" to the "" domain, we can run:


This is a somewhat dense line of code and may look confusing, but it makes more sense if we can break it down. Remember, emails[""] will retrieve the value associated with the key "", which is a list! So, we can use the append method directly on this list to add an element to it. Try it yourself to get comfortable with the concept. Are there any other methods you think might be useful to manipulate the values of this dictionary?

We might also want to print out the values of the dictionary in a presentable way. Just calling print(emails) at this point will give us this:

{'': ['annie', 'joseph', 'savitha', 'yeohee'], '': ['ben10', 'annie', 'dio'], '': ['joseph', 'hotmail', 'coda']}

This is certainly correct, but it doesn’t represent our information in a way that is very useful to us. Let’s instead print out all the items in a particular domain in the standard email format (such as For loops are helpful here!

domain = ""
for username in emails[domain]:
    print(username + "@" + domain)

You can get a lot out of dictionaries if you use them well. Let’s get a bit of practice.