Python Programming On Win32

Dictionaries

Python also offers a dictionary type. This is based on a hash table, and the lookup time is almost constant, irrespective of size. Dictionaries are enclosed in braces ({ }), and the keys and values are displayed separated by a colon. You can access and set their elements with a notation similar to list indexes:

>>> fur_colors = {}
>>> fur_colors['Tinky-Winky'] = 'purple'
>>> fur_colors['Dipsy'] = 'green'
>>> fur_colors['LaLa'] = 'yellow'
>>> fur_colors
{'Tinky-Winky': 'purple', 'Dipsy': 'green', 'LaLa': 'yellow'}
>>> fur_colors['LaLa']
'yellow'
>>>

Dictionaries have no natural order. They support some useful methods of searching, e.g., by keys, values, and whether or not a certain key is present:

>>> fur_colors.keys()
['Tinky-Winky', 'Dipsy', 'LaLa']
>>> fur_colors.values()
['purple', 'green', 'yellow']
>>> fur_colors.items()    # converts to a list of tuples
[('Tinky-Winky', 'purple'), ('Dipsy', 'green'), ('LaLa', 'yellow')]
>>> len(fur_colors)
3
>>> fur_colors.has_key('Po')
0
>>>

Lists and dictionaries together allow you to build more powerful data structures, such as sets or even database indexes, in few lines of code. The values in a dictionary can be anything, while the keys can be strings, numbers, or tuples of other values. In Chapter 6, we show how to construct some extremely useful utilities from dictionaries.