Workshop: Classes and OOP Basics (Optional, Self-Study)¶

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Six optional workshop problems, one per concept step, in a car scenario. Self-study: check python-workshop/solutions.

Step-01: Why Objects? Your First Class¶

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

Write a program with one class Car:

• Task-1: write the class with a class attribute wheels set to 4.
• Task-2: create two cars c1 and c2, and print each one's wheels.
• Task-3: print c1 itself, to see a plain object.

4
4
<__main__.Car 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 Car
class Car:
    wheels = 4

# Task-2: create two cars and print the shared wheels
c1 = Car()
c2 = Car()
print(c1.wheels)
print(c2.wheels)
# Task-3: print the object itself, which shows a memory address, not data
print(c1)

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

Concepts: __init__, self, instance attributes.

Write a program with one class Car:

• Task-1: write the class with an __init__ that takes a brand and a year.
• Task-2: create c1 (Mazda, 2020) and print its brand and year on one line.
• Task-3: create c2 (Tesla, 2023) and print its own brand and year on one line.

Mazda 2020
Tesla 2023

# Task-1: the class, with __init__ storing each value on self
class Car:
    def __init__(self, brand, year):
        self.brand = brand
        self.year = year

# Task-2: create c1 and print its data
c1 = Car("Mazda", 2020)
print(c1.brand, c1.year)

# Task-3: create c2, which keeps its own data
c2 = Car("Tesla", 2023)
print(c2.brand, c2.year)

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Step-03: Class Attributes vs Instance Attributes¶

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

Write a program with one class Car:

• Task-1: write the class with a class attribute wheels set to 4 and an __init__ taking brand and year.
• Task-2: create c1 (Mazda, 2020) and c2 (Tesla, 2023), print c1.wheels, then print Car.wheels straight from the class.
• Task-3: change c1.year to 2021, then print both cars' years.

4
4
2021
2023

# Task-1: the class, with a shared class attribute and per-object data
class Car:
    wheels = 4

    def __init__(self, brand, year):
        self.brand = brand
        self.year = year

# Task-2: both cars, then the class attribute read from the object and from the class
c1 = Car("Mazda", 2020)
c2 = Car("Tesla", 2023)
print(c1.wheels)
print(Car.wheels)

# Task-3: an instance attribute changes for c1 only
c1.year = 2021
print(c1.year)
print(c2.year)

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Step-04: Adding Behavior with Methods¶

Concepts: a method uses self, returning vs printing.

Write a program with one class Car:

• Task-1: write the class with an __init__ that takes a brand and a speed, a method accelerate that adds an amount to the speed, and a method status that RETURNS <brand> at <speed> km/h.
• Task-2: create c1 (Mazda, 40), accelerate it by 20, and print c1.status().

Mazda at 60 km/h

# Task-1: the class, with accelerate() that ACTS and status() that RETURNS
class Car:
    def __init__(self, brand, speed):
        self.brand = brand
        self.speed = speed

    def accelerate(self, amount):
        self.speed += amount

    def status(self):
        return f"{self.brand} at {self.speed} km/h"

# Task-2: create the car, accelerate it, then print what status() RETURNS
c1 = Car("Mazda", 40)
c1.accelerate(20)
print(c1.status())

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Step-05: Readable Objects: __str__ and __repr__¶

Concepts: __str__ friendly, __repr__ exact.

Write a program with one class Car:

• Task-1: write the class with an __init__ that takes a brand and a year, __str__ returning <brand> (<year>), and __repr__ returning Car(brand='<brand>', year=<year>).
• Task-2: create c1 (Mazda, 2020) and c2 (Tesla, 2023), and print c1.
• Task-3: print a list holding both cars.

Mazda (2020)
[Car(brand='Mazda', year=2020), Car(brand='Tesla', year=2023)]

# Task-1: the class, with both dunder methods
class Car:
    def __init__(self, brand, year):
        self.brand = brand
        self.year = year

    def __str__(self):
        return f"{self.brand} ({self.year})"

    def __repr__(self):
        return f"Car(brand={self.brand!r}, year={self.year!r})"

# Task-2: create both cars; print() uses __str__
c1 = Car("Mazda", 2020)
c2 = Car("Tesla", 2023)
print(c1)
# Task-3: a list of objects uses __repr__
print([c1, c2])

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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 asking the helper about an object the class method just built.

Write a program with one class Car:

• Task-1: write the class with an __init__ that takes a brand and a year, a @classmethod from_dict(cls, data) that builds a Car from a dict with keys brand and year, and a @staticmethod is_vintage(year) that returns True when year is below 1990.
• Task-2: print Car.is_vintage(1985).
• Task-3: build a car from {"brand": "Mazda", "year": 2020} using from_dict, and print its brand and year.
• Task-4: print whether that car is vintage, by passing its year to is_vintage.

True
Mazda 2020
False

# Task-1: the class, with the two decorated methods
class Car:
    def __init__(self, brand, year):
        self.brand = brand
        self.year = year

    @classmethod
    def from_dict(cls, data):
        return cls(data["brand"], data["year"])

    @staticmethod
    def is_vintage(year):
        return year < 1990

# Task-2: a static helper, no object needed
print(Car.is_vintage(1985))

# Task-3: an alternate constructor that builds a Car from a dict
c = Car.from_dict({"brand": "Mazda", "year": 2020})
print(c.brand, c.year)

# Task-4: the same static helper, now asked about the car we just built
print(Car.is_vintage(c.year))

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Workshop: Standard Library Essentials