20.1 Classes and OOP

Before you start

Work through the earlier fundamentals chapters. Use a Python 3 interpreter and run each example locally.

Core concepts

A class defines a type; an instance holds its own state. __init__ initializes an instance, and instance methods receive that instance as self. Prefer functions for simple transformations; use a class when related operations share meaningful state.

An instance attribute belongs to one object. A mutable class attribute is shared, which can surprise you. Composition builds an object from other objects; inheritance specializes a type and should preserve the parent’s expected behavior.

Worked example

class Server:
    def __init__(self, name, status):
        self.name = name
        self.status = status

    def is_healthy(self):
        return self.status == "healthy"

server = Server("web-01", "healthy")
print(server.is_healthy())

Mini lab

Create two Server objects. Change only the first status to critical. The second must remain healthy.

class Server:
    def __init__(self, name, status):
        self.name = name
        self.status = status

    def is_healthy(self):
        return self.status == "healthy"

first = Server("web-01", "healthy")
second = Server("web-02", "healthy")
first.status = "critical"
assert first.is_healthy() is False
assert second.is_healthy() is True

Knowledge check

When would you prefer a function over a class?

Common mistake

Do not introduce an inheritance hierarchy just to reuse a few lines. Start with small functions and composition.

Completion checklist

• Run the example and explain its output.
• Complete the mini lab without copying the solution.
• Test an edge case and explain how the code handles it.
• Answer the knowledge check in your own words.

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