The dict type – associating keys with values

Dictionaries – embodied in the dict type – are completely fundamental to the
way the Python language works, and are very widely used. A dictionary maps keys
to values, and in some languages it is known as a map or associative array. Let's look at how to create and use dictionaries in Python.

Literal dictionaries are created using curly braces containing key-value pairs. Each pair is separated by a comma, and each key is separated from its corresponding value by a colon. Here we use a dictionary to create a simple telephone directory:

>>> d = {'alice': '878-8728-922', 'bob': '256-5262-124', 
         'eve': '198-2321-787'}

We can retrieve items by key using the square brackets operator:

>>> d['alice']
'878-8728-922'

And we can update the value associated with a particular key by assigning through the square brackets:

>>> d['alice'] = '966-4532-6272'
>>> d
{'bob': '256-5262-124', 'eve': '198-2321-787', 
 'alice': '966-4532-6272'}

If we assign to a key that has not yet been added, a new entry is created:

>>> d['charles'] = '334-5551-913'
>>> d
{'bob': '256-5262-124', 'eve': '198-2321-787',
'charles': '334-5551-913', 'alice': '966-4532-6272'}

Be aware that the entries in the dictionary can't be relied upon to be stored in any particular order, and in fact the order that Python chooses may even change between runs of the same program. Similarly to lists, empty dictionaries can be created using empty curly braces:

>>> e = {}

This has been a very cursory look at dictionaries, but we'll be revisiting them in much more detail in Chapter 5, Exploring built-in Collection types.