Programming Fundamentals › Object-Oriented Programming

Method

A function that belongs to an object or class.

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Also known as: member function, instance method

A method is a function that belongs to a class or object. It works on that object’s data.

class Cart:
    def __init__(self):
        self.items = []

    def add(self, item):                 # a method
        self.items.append(item)

    def total(self):
        return sum(i.price for i in self.items)

cart = Cart()
cart.add(book)          # called on an object
cart.total()

The object is passed in automatically, and shows up as self (Python) or this (JavaScript, Java) inside the method (this and self).

Kinds

Kind	Belongs to	Example
Instance method	one object, can use its data	`cart.add(item)`
Static / class method	the class itself, no object needed	`Cart.from_json(text)` (static members)

Function or method?

If it needs the object’s data, make it a method. If it doesn’t use any, a plain function (or a static method) is often clearer. Not everything has to live in a class.

Common mistakes

Forgetting self as the first parameter in Python, and getting “takes 1 positional argument but 2 were given”.
Passing a method as a callback and losing its object. In JavaScript, button.onclick = obj.handle calls it with the wrong this (this binding).
Methods that do too much. Name them for one action (add, total), and keep them short.
A parent’s method can be replaced in a subclass (method overriding).