Dictionaries

The last data structure we will cover in this module is a dictionary. Dictionaries provide us with the ability to organize data in key-value pairs: while in lists and tuples we use an index to retrieve a value, in dictionaries we use a key.

To define a dictionary we enclose a sequence of comma-separated key: value pairs in curly brackets:

trips = {
    "Madrid": 2,
    "Paris": 1,
    "Berlin": 7,
    "London": 1,
}

Here we created a dictionary to store the number of times we visited european capitals. The keys in the dictionary are the cities "Madrid", "Paris", "London" and "Madrid", while the values are 2, 1, 7 and 1.

To retrieve a particular value from this dictionary, we use the corresponding key:

trips["Madrid"]

which returns the value 2.

One thing to note is that dictionaries can store any type of value, but the keys under which they are stored must be hashable values. This typically means that:

• Mutable values cannot be used as keys
• Immutable values can be used as keys, provided they do not contain mutable values. For instance, the tuple x = (["a", "b"], 1) is immutable, but it cannot be used as a key because contains a mutable value (the list ["a", "b"])

In the previous example we used strings (e.g. "Madrid") as keys, but we can use any other hashable value. Here we use integers as keys:

visits = {
    1: ["Paris", "London"],
    2: ["Madrid"],
    7: ["Berlin"],
}

The dictionary above uses the number of visits as keys and the cities visited as values.