Learning Python: Day 13

Chapter 6 – Dictionaries

can nest dictionaries inside lists, lists inside dictionaries, and even dictionaries inside other dictionaries
understanding dictionaries allows you to model real-world objects more accurately
for example, you can create a dictionary representing a person and then store as much information as you want about that person, such as their name, age, location, profession, etc
you can store any two kinds of information that can be matched up, such as a list of words and their meaning, a list of people’s names and their favorite numbers, a list of mountains and their elevations

A simple dictionary

can create a dictionary for a game featuring aliens with different colors and point values

alien.py

alien_0 = {‘color’: ‘green’, ‘points’: 5}

print(alien_0[‘color’])

print(alien_0[‘points’])

green

Working with dictionaries

dictionary – collection of key-value pairs
key – connected to a value, can be used to access the value associated with it
a key’s value can be a number, string, list, or another dictionary
in Python, a dictionary is wrapped in braces ({}) with a series of key-value pairs inside the braces

alien_0 = {‘color’: ‘green’, ‘points’: 5}

key-value pair – set of values associated with each other, every key is collected with its value by a colon
individual key-value pairs are connected by commas
simplest dictionary has one key-value pair

alien_0 = {‘color’: ‘green’}

Accessing values in a dictionary

give the name of the dictionary and then place the key inside a set of square brackets to get the value associated with a key

alien.py

alien_0 = {‘color’: ‘green’}

print(alien_0[‘color’])

green

you can have an unlimited number of key-value pairs

alien_0 = {‘color’: ‘green’, ‘points’: 5}

can access either value, if a player shoots down his alien, you can look up how many points they should earn

alien_0 = {‘color’: ‘green’, ‘points’: 5}

new_points = alien_0[‘points’]

print(f”You just earned {new_points} points!”)

You just earned 5 points!

if you run this code every time an alien is shot down, the alien’s point value will be retrieved

Adding new key-value pairs

dictionaries are dynamic structures, can add key-value pairs at any time
to add a new pair, give the name of the dictionary followed by the new key in square brackets, along with the new value
for example, adding the alien’s x- and y-coordinates to display the aline’s position on the screen

alien.py

alien_0 = {‘color’: ‘green’, ‘points’: 5}

print(alien_0)

alien_0[‘x_position’] = 0

alien_0[‘y_position’] = 25

print(alien_0)

{‘color’: ‘green’, ‘points’: 5}

{‘color’: ‘green’, ‘points’: 5, ‘x_position’: 0, ‘y_position’: 25}

dictionaries retain the order in which they were defined

Starting with an empty dictionary

sometimes better to start with an empty dictionary and define as you go

alien.py

alien_0 = {}

alien_0[‘color’] = ‘green’

alien_0[‘points’] = 5

print(alien_0)

{‘color’: ‘green’, ‘points’: 5}

typically you would use an empty dictionary when storing user-supplied data or when writing code that generates a large number of key-value pairs automatically

Modifying values in a dictionary

give the name of the dictionary with the key in square brackets and the new value if you want to modify a value in the dictionary

alien.py

alien_0 = {‘color’: ‘green’}

print(f”The alien is {alien_0[‘color’]}.”)

alien_0[‘color’] = ‘yellow’

print(f”The alien is now {alien_0[‘color’]}.”)

The alien is green.

The alien is now yellow.

we can track the position of an alien moving at different speeds

alien_0 = {‘x_position’: 0, ‘y_position’: 25, ‘speed’: ‘medium’}

print(f”Original position: {alien_0[‘x_position’]}”)

# Move the alien to the right.

# Determine how far to move the alien based on its current speed.

if alien_0[‘speed’] == ‘slow’:

….x_increment = 1

elif alien_0[‘speed’] == ‘medium’:

….x_increment = 2

else:

….# This must be a fast alien.

….x_increment = 3

….# The new position is the old position plus the increment.

alien_0[‘x_position’] = alien_0[‘x_position’] = x_increment

print(f”New position: {alien_0[‘x_position’]}”)

start by defining an alien with an initial x position and y position, and a speed of ‘medium’
we omitted color and point values for simplicity, but this would work if we included those key-value pairs
we print the original value of x_position to see how far the alien moves to the right
an if-elif-else chain determines how far the alien should move to the right and assign this value to the variable x_increment
if the alien’s speed is ‘slow’, it moves one unit to the right; if the speed is ‘medium’, it moves two units to the right; and if it’s ‘fast’, it moves three units to the right
once the increment has been calculated, it’s added to the value of x_position
the result is stored in the dictionary’s x_position
because it’s a medium-speed alien, its position shifts two units to the right

Original x_position: 0

New x-position: 2

by changing one value in the alien’s dictionary, you can change the overall behavior of the alien
for example, to turn this medium-speed alien into a fast alien, you would add this line

alien_0[‘speed’] = ‘fast’

the if-elif-else block would then assign a larger value to x_increment the next time the code runs

Removing key-value pairs

when you no longer need a piece of information that’s stored in a dictionary, you can use the del statement to completely remove a key-value pair
all del needs is the name of the dictionary and the key that you want to remove
for example, let’s remove the key ‘points’ from the alien_0 dictionary, along with its value

alien.py

alien_0 = {‘color’: ‘green’, ‘points’: 5}

print(alien_0)

del alien_0[‘points’]

print(alien_0)

del statement tells Python to delete the key ‘points’ from the dictionary alien_0
to remove the value associated with that key as well
the output shows that the key ‘points’ and its value of 5 are deleted from the dictionary, but the rest of the dictionary is unaffected:

{‘color’: ‘green’, ‘points’: 5}

{‘color’: ‘green’}

the key-value pair is removed permanently

A dictionary of similar objects

the previous example involved storing different kinds of information about one object
you can also use a dictionary to store one kind of information about many objects
for example, a dictionary can store the results fo a simple poll

favorite_languages.py

favorite_languages = {

….’jen’: ‘python’,

….’sarah’: ‘c’,

….’edward’: ‘rust’,

….’phil’: ‘python’,

….}

we’ve broken a larger dictionary into several lines
each key is the name of a person who responded to the poll
each value is their language choice
when you need more than one line to define a dictionary, press enter after the opening brace, indent on the next line (four spaces) and write the first key-valye pair, followed by a comma
press enter, your text editor should indent all subsequent key-value pairs automatically
finally, add a closing brace on a new line after the last key-value pair once you’ve finished defining the dictionary, and indent it one level
can use dictionary to look up a name of a person and their favorite language

favorite_languages.py

favorite_languages = {

….’jen’: ‘python’,

….’sarah’: ‘c’,

….’edward’: ‘rust’,

….’phil’: ‘python’,

….}

language = favorite_languages[‘sarah’].title()

print(f”Sarah’s favorite language is {language}.”)

Sarah’s favorite language is C.

Using get() to access values

using keys in square brackets to retrieve the value you’re interested in will cause an error if the key doesn’t exist

alien_no_points.py

alien_0 = {‘color’: ‘green’, ‘speed’: ‘slow’}

print(alien_0[‘points’]

results in a traceback error
get() method – sets a default value that will be returned if the requested key doesn’t exist
get() method requires a key as a first argument, as a second optional argument, you can pass the value to be returned if the key doesn’t exist

alien_0 = {‘color’: ‘green’, ‘speed’: ‘slow’}

point_value = alien_0.get(‘points’, ‘No point value assigned.’)

print(point_value)

if the key ‘points’ exists, you’ll get a value
if it doesn’t, you’ll get the default value

No point value assigned.

consider using get() method instead of square brackets if there’s a chance the key might not exist

Looping through a dictionary

can loop through all key-value [airs in a dictionary, through its keys or values

Looping through all key-value pairs

example, dictionary storing user’s username, first name, and last name

user.py

user_0 = {

….’username’: ‘efermi’,

….’first’: ‘enrico’,

….’last’: ‘fermi’,

}

can access any single piece of information about user_0, but what if you wanted to see everything stored in this user’s dictionary
use a for loop

user_0 = {

….’username’: ‘efermi’,

….’first’: ‘enrico’,

….’last’: ‘fermi’,

}

for key, value in user_0.items():

….print(f”\nKey: {key}”)

….print(f”Value: {value}”)

create names for the two variables that hold the key and value in each key-value pair to write a for loop in a dictionary
can choose any names
could work if you used abbreviations for the variable names

for k, v in user_0.items()

second half of the for statement includes the name of the dictionary followed by the method items(), which returns a sequence of key-value pairs
for loop then assigns each pairs to the provided variables

Key: username

Value: efermi

Key: first

Value: enrico

Key: last

Value: fermi

back to the previous dictionary for favorite programming language, because the keys always refer to a person’s name and the value is always a language, we’ll use the variables name and language in the loop to make it easier to follow what’s happening

Learning Python: Day 13

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