All of the compound data types we have studied in detail so far — strings, lists, and tuples — are sequential collections. This means that the items in the collection are ordered from left to right and they use integers as indices to access the values they contain.
Dictionaries are a different kind of collection. They are Python’s built-in mapping type.
One way to create a dictionary is to start with the empty dictionary and add key-value pairs.
A dictionary consists of a collection of key-value pairs. Each key-value pair maps the key to its associated value. You can define a dictionary by enclosing a comma-separated list of key-value pairs in curly braces ({}). A colon (:) separates each key from its associated value:
An item has a key and a corresponding value that is expressed as a pair (key: value). While the values can be of any data type and can repeat, keys must be of immutable type (string, number or tuple with immutable elements) and must be unique.