Python Methods

What is a Method?

• A method is a function that acts on an instance of a class
• Unlike regular functions, methods can access all attributes associated with a specific instance
• All instance methods require self as their first parameter
• self stores the instance the method is acting on — it's passed automatically, you don't pass it explicitly

Defining a Method

Methods are defined inside a class using the def keyword, indented under the class body:

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

    def say_hello(self):
        print(f"Hello, {self.name}")

p1 = Person("Tim")
p1.say_hello()   # Output: Hello, Tim

__init__ is itself a special method (the initializer). It follows the same rules as any other method.

Setter and Getter Methods

Setter

A method that assigns a value to an attribute:

def set_age(self, age):
    self.age = age

Getter

A method that returns the value of an attribute:

def get_age(self):
    return self.age

Guide: In Python, getters and setters are largely unnecessary because you can access attributes directly (e.g. p1.age). They exist in other languages for encapsulation.

Best Practice: Define All Attributes in __init__

Always declare every attribute you plan to use inside __init__, even if you don't have a value for it yet. Use None or a sensible default:

class Person:
    def __init__(self, name):
        self.name = name
        self.age = None     # not yet set, but declared
        self.weight = 0
        self.height = 0

Why? If you try to access an attribute before it's been created, Python raises an AttributeError.

Practical Example — Counter Class

class Counter:
    def __init__(self):
        self.count = 0
        self.locked = False

    def increment(self):
        if self.locked:
            raise Exception("The counter is locked!")
        self.count += 1

    def decrement(self):
        if self.locked:
            raise Exception("The counter is locked!")
        self.count -= 1

    def print_count(self):
        print(f"The current count is {self.count}")

    def toggle_lock(self):
        self.locked = not self.locked   # flips True ↔ False

Usage

counter = Counter()
counter.increment()
counter.increment()
counter.decrement()
counter.print_count()   # The current count is 1

counter.toggle_lock()
counter.decrement()     # Raises Exception: The counter is locked!

Each Instance is Independent

Calling a method on one instance does not affect another instance:

counter1 = Counter()
counter2 = Counter()

counter1.toggle_lock()   # only counter1 is locked
counter2.increment()     # this still works fine

Each instance maintains its own separate copy of all attributes.

Key Takeaways & Recap