4.1 Classes and Instances | Code Scholars Tutoring

How it works

A class is a template; an instance is one object built from it. The initialiser runs at construction and sets up instance state.

Give every attribute a value in the initialiser so no instance starts half-built.

Common error

Forgetting the instance parameter on a method, which produces a confusing argument-count error.

Worked example

Python

class Counter:
    def __init__(self, start):
        self.value = start
    def increment(self):
        self.value += 1

a = Counter(2)
b = Counter(10)
a.increment()
print(a.value, b.value)

Output

3 10

Each instance holds its own value. self identifies the instance receiving a method call; calling a.increment changes a rather than b.

Exercises

4.1.1 · Explain

What role does __init__ play when Counter(5) is evaluated?

Show solution

It initializes the new instance state using the supplied argument. It should not return an alternate object or a non-None result.

4.1.2 · Write code

Create a Box class whose constructor stores width and whose area method returns width squared. Show Box(4).area().

Show solution

Store instance state through self.

One valid solution

class Box:
    def __init__(self, width):
        self.width = width
    def area(self):
        return self.width ** 2

print(Box(4).area())

Expected output

Open Python compiler