Erle Robotics Learning Python GitBook Free

Dictionaries basics

A dictionary is similar to a list, but you access values by looking up a key instead of an index. A key can be any string or number. Dictionaries are enclosed in curly braces, like so:

d = {'key1' : 1, 'key2' : 2, 'key3' : 3}

This is a dictionary called d with three key-value pairs. The key 'key1' points to the value 1, 'key2' to 2, and so on.

Dictionaries are great for things like phone books (pairing a name with a phone number), login pages (pairing an e-mail address with a username)...

Accessing a dictionary

Note that accessing dictionary values by key is just like accessing list values by index.

Practice 1

Given this dictinary:

residents = {'Puffin' : 104, 'Sloth' : 105, 'Burmese Python' : 106}

Print the value stored under the 'Sloth' key:

>>> residents = {'Puffin' : 104, 'Sloth' : 105, 'Burmese Python' : 106}
>>> print residents['Sloth']

Adding elements to a dictionary

Like Lists, Dictionaries are "mutable". This means they can be changed after they are created. One advantage of this is that we can add new key/value pairs to the dictionary after it is created like so:

dict_name[new_key] = new_value

An empty pair of curly braces {} is an empty dictionary, just like an empty pair of[]is an empty list.

The length len() of a dictionary is the number of key-value pairs it has. Each pair counts only once, even if the value is a list. (That's right: you can put lists inside dictionaries!)

Practice 2

Add three elements to menu{}. Print the number of elements in menu and the menu itselfs.

>>> menu = {} # Empty dictionary
>>> menu['Sunday']=16.78
>>> print menu['Sunday']
>>> menu['Monday']=6.78
>>> print menu['Monday']
>>> menu['Tuesday']= 9.98
>>> print menu['Tuesday']
>>> print "There are " + str(len(menu)) + " items on the menu."
There are 3 items on the menu.
>>> print menu
{'Sunday': 16.78, 'Tuesday': 9.98, 'Monday': 6.78}

Removing elements from a dictionary and associating values with keys.

Because dictionaries are mutable, they can be changed in many ways. Items can be removed from a dictionary with the del command:

del dict_name[key_name]

will remove the key key_name and its associated value from the dictionary.

A new value can be associated with a key by assigning a value to the key, like so:

dict_name[key] = new_value
Practice 3

Given the dictionary:

# key - animal_name : value - location
zoo_animals = { 'Unicorn' : 'Cotton Candy House',
'Sloth' : 'Rainforest Exhibit',
'Bengal Tiger' : 'Jungle House',
'Atlantic Puffin' : 'Arctic Exhibit',
'Rockhopper Penguin' : 'Arctic Exhibit'}
  • Delete the 'Sloth' and 'Bengal Tiger' items from zoo_animals using del.

  • Set the value associated with 'Rockhopper Penguin' to anything other than 'Arctic Exhibit.

>>> zoo_animals = { 'Unicorn' : 'Cotton Candy House',
... 'Sloth' : 'Rainforest Exhibit',
... 'Bengal Tiger' : 'Jungle House',
... 'Atlantic Puffin' : 'Arctic Exhibit',
... 'Rockhopper Penguin' : 'Arctic Exhibit'}
>>> del zoo_animals['Sloth']
>>> del zoo_animals['Bengal Tiger']
>>> print zoo_animals
{'Atlantic Puffin': 'Arctic Exhibit', 'Rockhopper Penguin': 'Arctic Exhibit', 'Unicorn': 'Cotton Candy House'}
>>> zoo_animals['Rockhopper Penguin']='Arctic Dance'
>>> print zoo_animals
{'Atlantic Puffin': 'Arctic Exhibit', 'Rockhopper Penguin': 'Arctic Dance', 'Unicorn': 'Cotton Candy House'}