Practice: Classes and OOP Basics¶

View practice code on GitHub Get the Video Course

Six practice problems, one per concept step; the numbers line up. Build each yourself, then check python-practice/solutions.

These problems use a music app theme (a Song class with a shared app name, plays, and readable songs), a different domain from the Pyzon Store you build in the concept files, so you apply each idea instead of copying the demo.

Step-01: Why Objects? Your First Class¶

Concepts: a class as a blueprint, a class attribute, creating objects.

Write a program with one class Song:

• Task-1: write the class with a single class attribute app set to Tunefy.
• Task-2: create two songs s1 and s2 from it, and print each one's app.
• Task-3: print s1 itself, to see what a plain object looks like.

Tunefy
Tunefy
<__main__.Song object at 0x...>

The number after 0x is a memory address, so it is different on every run. Yours will not match this one.

# Task-1: the class, with one value shared by every Song
class Song:
    app = "Tunefy"

# Task-2: create two songs and print the shared app
s1 = Song()
s2 = Song()
print(s1.app)
print(s2.app)

# Task-3: print the object itself, which shows a memory address, not data
print(s1)

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Step-02: Give Each Object Its Own Data (__init__ and self)¶

Concepts: __init__, self, instance attributes.

Write a program with one class Song:

• Task-1: write the class with an __init__ that takes a title and an artist and stores each on self.
• Task-2: create s1 with title Yesterday and artist The Beatles, and print its title and artist on one line.
• Task-3: create s2 with title Hello and artist Adele, and print its own title and artist on one line.

Yesterday The Beatles
Hello Adele

# Task-1: the class, with __init__ storing each value on self
class Song:
    def __init__(self, title, artist):
        self.title = title
        self.artist = artist

# Task-2: create s1 and print its data
s1 = Song("Yesterday", "The Beatles")
print(s1.title, s1.artist)

# Task-3: create s2, which keeps its own data
s2 = Song("Hello", "Adele")
print(s2.title, s2.artist)

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Step-03: Class Attributes vs Instance Attributes¶

Concepts: a class attribute shared by all, instance attributes per object, precedence.

Write a program with one class Song:

• Task-1: write the class with a class attribute app set to Tunefy and an __init__ taking a title and an artist.
• Task-2: create s1 (Yesterday, The Beatles) and s2 (Hello, Adele), print s1.app, then print Song.app straight from the class.
• Task-3: give s2 its own app value MyTunes, then print s2.app, s1.app and Song.app, so the instance attribute wins for s2 while the others stay Tunefy.

Tunefy
Tunefy
MyTunes
Tunefy
Tunefy

# Task-1: the class, with a shared class attribute and per-object data
class Song:
    app = "Tunefy"

    def __init__(self, title, artist):
        self.title = title
        self.artist = artist

# Task-2: both songs, then the class attribute read from the object and from the class
s1 = Song("Yesterday", "The Beatles")
s2 = Song("Hello", "Adele")
print(s1.app)
print(Song.app)

# Task-3: an instance attribute on s2 wins for s2 only
s2.app = "MyTunes"
print(s2.app)
print(s1.app)
print(Song.app)

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Step-04: Adding Behavior with Methods¶

Concepts: a method uses self, a method changes data, returning vs acting.

Write a program with one class Song:

• Task-1: write the class with an __init__ that takes a title and an artist and sets self.plays to 0, a method play that adds 1 to self.plays, and a method label that RETURNS the text <title> by <artist>.
• Task-2: create s1 for Yesterday by The Beatles, play it 3 times, and print s1.plays.
• Task-3: print s1.label().

3
Yesterday by The Beatles

# Task-1: the class, with play() that ACTS and label() that RETURNS
class Song:
    def __init__(self, title, artist):
        self.title = title
        self.artist = artist
        self.plays = 0

    def play(self):
        self.plays += 1

    def label(self):
        return f"{self.title} by {self.artist}"

# Task-2: create the song, play it three times, then print the changed counter
s1 = Song("Yesterday", "The Beatles")
s1.play()
s1.play()
s1.play()
print(s1.plays)

# Task-3: print what label() RETURNS
print(s1.label())

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Step-05: Readable Objects: __str__ and __repr__¶

Concepts: __str__ for a friendly printout, __repr__ for the exact one; both are dunder methods, so the name must match exactly.

Write a program with one class Song:

• Task-1: write the class with an __init__ that takes a title and an artist, __str__ returning <title> by <artist>, and __repr__ returning Song(title='<title>', artist='<artist>').
• Task-2: create s1 (Yesterday, The Beatles) and s2 (Hello, Adele), and print s1.
• Task-3: print a list holding both songs.

Yesterday by The Beatles
[Song(title='Yesterday', artist='The Beatles'), Song(title='Hello', artist='Adele')]

# Task-1: the class, with both dunder methods
class Song:
    def __init__(self, title, artist):
        self.title = title
        self.artist = artist

    def __str__(self):
        return f"{self.title} by {self.artist}"

    def __repr__(self):
        return f"Song(title={self.title!r}, artist={self.artist!r})"

# Task-2: create both songs; print() uses __str__
s1 = Song("Yesterday", "The Beatles")
s2 = Song("Hello", "Adele")
print(s1)

# Task-3: a list of objects uses __repr__
print([s1, s2])

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Step-06: Class Methods and Static Methods (@classmethod and @staticmethod)¶

Concepts: the decorators @classmethod (an alternate constructor: it receives the class as cls) and @staticmethod (a related helper: it takes neither self nor cls), and using both together.

Write a program with one class Song:

• Task-1: write the class with an __init__ that takes a title, an artist and a duration in seconds, a @classmethod from_dict(cls, data) that builds a Song from a dict with keys title, artist and duration, and a @staticmethod is_valid_duration(duration) that returns True when duration is above 0.
• Task-2: print Song.is_valid_duration(125).
• Task-3: take the two dicts {"title": "Yesterday", "artist": "The Beatles", "duration": 125} and {"title": "Silence", "artist": "Unknown", "duration": 0}. For each one, build the song with from_dict and print its title and duration only when is_valid_duration says the duration is valid, and print Duration is not valid when it is not.

True
Yesterday 125
Duration is not valid

# Task-1: the class, with the two decorated methods
class Song:
    def __init__(self, title, artist, duration):
        self.title = title
        self.artist = artist
        self.duration = duration

    @classmethod
    def from_dict(cls, data):
        return cls(data["title"], data["artist"], data["duration"])

    @staticmethod
    def is_valid_duration(duration):
        return duration > 0

# Task-2: a static helper, no object needed
print(Song.is_valid_duration(125))

# Task-3: the helper checks first, then the alternate constructor builds the Song
songs = [{"title": "Yesterday", "artist": "The Beatles", "duration": 125},
         {"title": "Silence", "artist": "Unknown", "duration": 0}]
for data in songs:
    if Song.is_valid_duration(data["duration"]):
        s1 = Song.from_dict(data)
        print(s1.title, s1.duration)
    else:
        print("Duration is not valid")

Pages for this step: Concept | Practice | Workshop

Python Files for this step: Concept | Practice | Practice Solutions | Workshop | Workshop Solutions

Practice: Virtual Environments and Packaging