Classes

Objects and classes

When talking about OOP, its hard not to interchange the words class and object. In truth, the difference between them is quite subtle: an object is an instance (or “a specific example”) of a class. For example, "hello!" is an instance of a str, 3 is an instance of an int, and 54.12 is an instance of a float.

While we’re learning, we’ll try to be precise with our language: we will use the word class when referring to the type of variable, while we will use the word object to the specific variable itself. This will become clearer as we progress.

Defining a class

Let’s start by defining a class. A class definition consists of the class name, and a constructor __init__, where attributes are typically given values.

class Person():
    def __init__(self):
        self.name = ""

Here our class Person has one attribute name and no methods.

Create a new file/document in Thonny and save it locally with the file name lab7p1.py. Add the course header with the relevant information.

Initializing a class

Below is a more complete example (add it to your lab file):

class Person():
    def __init__(self, name):
      self.name = name
      
    def sayHi(self):
      print("Hi, my name is", self.name) 
def main():
  my_friend = Person("Bob")
  my_friend.sayHi()
if __name__ == "__main__":
    main()

Instantiating is the moment in which we call the constructor and assign it’s value to a variable. The instantiation of the class happens when we say my_friend = Person(Bob). Notice that this instantiation is outside the class.

In the first example, the constructor did not take name as a parameter. This would cause issues if someone calls the sayHi() method before actually having a name stored. To enforce all people to have names, we add a required parameter to the constructor.

To really drive home what’s happening here, let’s create three different instances, and then call the .sayHi() method on each of them:

my_friend_1 = Person("Alice")
my_friend_2 = Person("Bob")
my_friend_3 = Person("Chloe")
my_friend_1.sayHi()
my_friend_2.sayHi()
my_friend_3.sayHi()

Even though the .sayHi() method was only defined in one place (the definition of the Person class), it has slightly different behavior on each instance. That’s because each Person has their own name attribute, and that is what the .sayHi() method is referencing when it gets called on each instance.

Another cool thing about attributes? They can be any data type - even other classes!

• Add the following attributes to your Person class:
  • age (initialized to a random integer between 1 and 99)
  • favoriteColor (initialized by passing an additional parameter to the constructor)
• Modify the .sayHi() method to include these attributes in the printed message, e.g. Hi, my name is Kacey, I am 21 years old and my favorite color is green.

Begin by downloading the short answer starter file (click this link), and save it locally. Open the file with either Thonny or a text editor. Answer the following short answer question (1-2 English sentences.):

• SA Question #1: What other attributes would you add to the person class?
• SA Question #2: What other methods would you add to the person class?

Song Class

We will begin by defining a class to store the songs. Write the song class given the following specification (i.e., what needs to go in it). Be sure to comment your code as you go.

Class Song: A class that represents a single song.

Attributes:

• title: Song title as a string
• url: Song duration as a string
• duration: Song url as a string

Contructor and Methods:

• constructor (title, url, duration): Takes three strings and saves them as attributes.
• printInfo ( ): Prints the information about the song in a friendly way (e.g. “Help - The Beatles 2:19”).
• play ( ): Opens a web browser and plays the song. The program will sleep for the duration of the song.

Create a main function (outside your Song class). We will use this function to test our class and will expand on it further in the next step.

You must test each method once you create it. You can write a helper functions (outside your class), to test it. Add the following code above your main function.

def testSongPrintInfo():
    s1 = Song("Help","https://www.youtube.com/watch?v=2Q_ZzBGPdqE", "02:19")
    s1.printInfo()

Then call the following code from inside your main function:

testSongPrintInfo()

Once your program works for printInfo, add the play method. Review our previous lab to recall how to play a song.

Add an additional test method entitled testSongPlay(), to test your play method.

PlaylistManager Class

Just having individual song objects isn’t all that helful because we still need to collect them somehow and be able to play music. The PlaylistManager is a class that is responsible for keeping a list of the songs a user wants to store and play. Think of it like an iPod. You only need one of them, it initially comes out of the box with no songs stored in it. It has the ability to add a new song, play a song, shuffle songs, delete a song, list all the songs, etc. Our PlaylistManager will have only some of these abilities but you could easily keep working on this to make it have more and more of them.

Class PlaylistManager: Holds a list of Song objects in a playlist and provides methods for performing the operations on the playlist.

Attributes:

• song_list: list of Song objects

Contructor and Methods:

• constructor ( ): initializes an empty list.
• addSong (title, url, duration): creates a Song object and adds it to the playlist.
• printAll( ): prints all the Songs in the playlist each on a separate line preceded by a number. Note: This should use the Song.print_info method.
• playAll ( ): play all the Songs in the playlist in sequence

For example, the print_all() function may display:

1. All in Good Time - Ron Sexsmith 3:45
2. Help - The Beatles 2:19
3. Walkin Blues - Robert Johnson 3:02

Again, only write one method at a time and test it individually (add helper methods). It is more important to write and test printAll, before playAll because it will reveal basic bugs prior to trying to play songs.

To wrap up today’s lab, finish the main() function below. It should create an instance of your PlaylistManager, allow the user to add songs until they say they’re done, and then play the songs in the playlist. Use the methods you defined in your PlaylistManager and Song classes to do most of the work!

def main():
  # Instantiate PlaylistManager
  playlistMgr = PlaylistManager()
  keepGoing = ""
  while(keepGoing != "DONE"):
    
    # Ask for info about song
    title = input("Song title: ")
    url = input("YouTube video: ")
    duration = input("Video duration (MM:SS): ")
    
    ## TODO: Add the Song to the PlaylistManager
    
    keepGoing = input("Add another song? YES to continue, DONE to end: ")
  
  ## TODO: when done adding songs print all the songs
  ## TODO: after printing all songs, play them all.
if __name__ == "__main__":
  main()

Note: In this case, main does not have access to any of the song objects. This is an example of encapsulation.