Use curly braces to define a dictionary. Use colons to
connect keys and values, and use commas to separate
individual key-value pairs.
alien_0 = {'color': 'green', 'points': 5}
Covers Python 3 and Python 2
You can loop through a dictionary in three ways: you can
loop through all the key-value pairs, all the keys, or all the
values.
A dictionary only tracks the connections between keys
and values; it doesn't track the order of items in the
dictionary. If you want to process the information in order,
you can sort the keys in your loop.
Looping through all key-value pairs
# Store people's favorite languages.
fav_languages = {
'jen': 'python',
'sarah': 'c',
'edward': 'ruby',
'phil': 'python',
}
# Show each person's favorite language.
for name, language in fav_languages.items():
print(name + ": " + language)
Looping through all the keys
# Show everyone who's taken the survey.
for name in fav_languages.keys():
print(name)
Looping through all the values
# Show all the languages that have been chosen.
for language in fav_languages.values():
print(language)
Looping through all the keys in order
# Show each person's favorite language,
# in order by the person's name.
for name in sorted(fav_languages.keys()):
print(name + ": " + language)
Python's dictionaries allow you to connect pieces of
related information. Each piece of information in a
dictionary is stored as a key-value pair. When you
provide a key, Python returns the value associated
with that key. You can loop through all the key-value
pairs, all the keys, or all the values.
To access the value associated with an individual key give
the name of the dictionary and then place the key in a set of
square brackets. If the key you're asking for is not in the
dictionary, an error will occur.
You can also use the get() method, which returns None
instead of an error if the key doesn't exist. You can also
specify a default value to use if the key is not in the
dictionary.
Getting the value associated with a key
alien_0 = {'color': 'green', 'points': 5}
print(alien_0['color'])
print(alien_0['points'])
Getting the value with get()
alien_0 = {'color': 'green'}
alien_color = alien_0.get('color')
alien_points = alien_0.get('points', 0)
print(alien_color)
print(alien_points)
You can modify the value associated with any key in a
dictionary. To do so give the name of the dictionary and
enclose the key in square brackets, then provide the new
value for that key.
Modifying values in a dictionary
alien_0 = {'color': 'green', 'points': 5}
print(alien_0)
# Change the alien's color and point value.
alien_0['color'] = 'yellow'
alien_0['points'] = 10
print(alien_0)
You can remove any key-value pair you want from a
dictionary. To do so use the del keyword and the dictionary
name, followed by the key in square brackets. This will
delete the key and its associated value.
Deleting a key-value pair
alien_0 = {'color': 'green', 'points': 5}
print(alien_0)
del alien_0['points']
print(alien_0)
You can store as many key-value pairs as you want in a
dictionary, until your computer runs out of memory. To add
a new key-value pair to an existing dictionary give the name
of the dictionary and the new key in square brackets, and
set it equal to the new value.
This also allows you to start with an empty dictionary and
add key-value pairs as they become relevant.
alien_0 = {'color': 'green', 'points': 5}
alien_0['x'] = 0
alien_0['y'] = 25
alien_0['speed'] = 1.5
Adding to an empty dictionary
alien_0['color'] = 'green'
alien_0['points'] = 5
Try running some of these examples on pythontutor.com.
You can find the number of key-value pairs in a dictionary.
Finding a dictionary's length
num_responses = len(fav_languages)