Class Attributes and Methods Lab

Learning Goals

• Use class attributes and methods to write durable and powerful code.
  • Store and access song data using class attributes and methods.
• Accomplish complex programming tasks using knowledge from previous modules.

---

Key Vocab

• Attribute: variables that belong to an object.
• Constant: variable whose value cannot be changed.
• Instance: one specific working copy of a class. It is created when a class's __init__ method is called.
• Class: a bundle of data and functionality. Can be copied and modified to accomplish a wide variety of programming tasks.
• Static: an attribute or method that cannot manipulate the class or instance it belongs to.
• Exception: an error that occurs during the execution of a program. Exceptions can be anticipated and handled without disrupting the execution of the program.

---

Introduction

In this lab, we'll be dealing with a Song class. The Song class can produce individual songs. Each song has a name, an artist and a genre. We need our Song class to be able to keep track of the number of songs that it creates.

Song.count
# => 30

We need our Song class to be able to show us all of the artists of existing songs:

Song.artists
# ["Jay-Z", "Drake", "Beyonce"]

We need our Song class to be able to show us all of the genres of existing songs:

Song.genres
# => ["Rap", "Pop"]

We also need our Song class to be able to keep track of the number of songs of each genre it creates.

In other words, calling:

Song.genre_count

Should return something like this;

{"Rap": 5, "Rock": 1, "Country": 3}

Lastly, we want our Song class to reveal to us the number of songs each artist is responsible for.

Song.artist_count
# {"Beyonce": 17, "Jay-Z": 40}

We'll accomplish this with the use of class attributes and class methods.

---

Instructions

Define your Song class such that an individual song is initialized with a name, artist and genre.

ninety_nine_problems = Song("99 Problems", "Jay-Z", "Rap")

ninety_nine_problems.name
# "99 Problems"

ninety_nine_problems.artist
# "Jay-Z"

ninety_nine_problems.genre
# "Rap"

Create a class attribute, count. We will use this attribute to keep track of the number of new songs that are created from the Song class. Set this attribute equal to 0.

At what point should we increment our count of songs? Whenever a new song is created. Your __init__ method should call a class method add_song_to_count() that increments the value of count by one.

Next, define the following class methods:

add_to_genres(): adds any new genres to a class attribute genres, a list. This list should contain only unique genres — no duplicates! Think about what you'll need to do to get this method working:

• You'll need a class attribute, let's call it genres, that is equal to an empty list.
• When should you add genres to the array? Whenever a new song is created. Your __init__ method should add the genre of the song being created to the genres list. All genres should be added to the list. Control for duplicates when you code your add_to_genres class method, not when you add genres to the original genres list. We will want to know how many songs of each genre have been created. We'll revisit that job a little later on.

add_to_artists(): adds any new artists to a class attribute artists, a list. This list should only contain unique artists, just like the genres class attribute. Once again, thnk about what you need to do to implement this behavior:

• You'll need a class attribute, artists, that is equal to an empty list.
• When should you add artists to this array? Whenever a new song is initialized. Your __init__ method should add artists to the artists list. All artists should be added to the list. Control for duplicates when you code your add_to_artists() class method, not when you add artists to the original artists list. We will want to know how many songs each have been assigned to each artist. We'll revisit that job a little later on when we write our add_to_artist_count() method.

add_to_genre_count(): adds to a class attribute genre_count, a dictionary in which the keys are the names of each genre. Each genre name key should point to a value that is the number of songs that have that genre.

Song.genre_count
# {"Rap": 5, "Rock": 1, "Country": 3}

This manner of displaying numerical data is called a histogram. How will you create your histogram? There are a few ways!

• You can need to iterate over the genres list and populate a dictionary with the key/value pairs. You will need to check to see if the hash already contains a key of a particular genre. If so, increment the value of that key by one, otherwise, create a new key/value pair.

add_to_artist_count(): creates a histogram similar to the one above, but for artists rather than genres.

---

Resources

• Python Documentation
• Classes - Python
• Python Class Attributes: An Overly Thorough Guide - Toptal
• Python's Instance, Class, and Static Methods Demystified - Real Python
• The Factory Method Pattern and Its Implementation in Python - Real Python