Dictionary Basics

In this section, you will learn a new Python data structure called a Dictionary.

What is it? Why do we need it? How do you use it?

What is a Dictionary?

So far, you've used Lists to manage collections of items.

In a list, items are ordered, and you use numeric indices to access them:

countries = ["Kenya", "Ghana", "Ethiopia", "Zimbabwe"]
countries[2] # "Ethiopia"

A dictionary, like a list, stores a collection of values. Instead of keeping them in order and accessing them with indices, a Dictionary has a key for each value.

capitals = {
  "Kenya": "Nairobi",
  "Ghana": "Accra",
  "Ethiopia": "Addis Ababa",
  "Zimbabwe": "Harare",
}
capitals["Kenya"] # "Nairobi"

Each value has a key, and you can access the value using the key.

Each key is associated with a single value. As in Lists, values can have any type. Keys are often strings, but can have other values too.

The association of a key and a value is called a "key-value pair" or sometimes an "item".

Mathematically speaking, a dictionary represents a mapping from keys to values. You can also say that each key “maps to” a value.

As an example, we’ll build a dictionary that maps from English to Spanish words, so the keys and the values are all strings. I encourage you to open a Python repl and run each of these commands yourself as you read. You'll learn it better if you are active, and it gives you a chance to explore.

Let's start with basic commands.

Creating dictionaries and adding items

>>> english_to_spanish = {}
>>> english_to_spanish
{}

The squiggly-brackets, {}, represent an empty dictionary.

To add items to the dictionary, you can use square brackets:

>>> english_to_spanish['one'] = 'uno'

This adds an item to the dictionary that maps from the key 'one' to the value 'uno'. If we show the dictionary again, we see the key-value pair, with a colon between the key and value:

>>> english_to_spanish
{'one': 'uno'}

This output format is also a valid syntax for creating a dictionary. Instead of adding items one by one, we can create a dictionary with three items:

>>> english_to_spanish = {'one': 'uno', 'two': 'dos', 'three': 'tres'}

Indexing and Order

If you print english_to_spanish, you might be surprised:

>>> english_to_spanish
{'one': 'uno', 'three': 'tres', 'two': 'dos'}

The order of the key-value pairs might not be the same as you entered!

In general, the order of items in a dictionary is unpredictable.

But, that’s no problem, since elements of a dictionary are not indexed by numbers. Instead, you use the keys to look up the corresponding values:

>>> english_to_spanish['two']
'dos'

The key 'two' maps to the value 'dos'. The order of the items doesn’t matter.

Here is a video that demonstrates the basics of dictionaries:

Missing Items

If you try to access a key that isn’t in the dictionary, you get an exception:

>>> english_to_spanish['four']
KeyError: 'four'

How many items in a Dictionary

The len function returns the number of key-value pairs:

>>> len(english_to_spanish)
3

Checking in the dictionary

The in operator works on dictionaries, too; it tells you whether a value appears as a key (appearing as a value does not count).

>>> 'one' in english_to_spanish
True
>>> 'uno' in english_to_spanish
False

To see whether something appears as a value in a dictionary, you can use the method values(), which returns a collection of values, and the in operator:

>>> 'uno' in english_to_spanish.values()
True

Try it: Dictionaries

Practice creating Dictionaries, accessing items, and updating values.

Try to accomplish the following:

Use ints and floats as Dictionary values.

Use an int and as float as a key. Are 3 and 3.0 the same key? What about 3 and 3.1?

How do you update a value that's already in a Dictionary?

How do you delete a key and value in a Dictionary?

What happens if you try to access a key after it has been deleted?

Further Experimentation

Once you've explored some of the basics, try to figure out some more complicated syntax:

Add a List as a value in a Dictionary

Add a Dictionary as a value in a Dictionary

If you store a Dictionary as a value in another Dictionary, how can you access the inner Dictionary's items?

Can you use a List as a key in a Dictionary?

Programming 1