Classes Writing a Python Class

Learning objective: By the end of this lesson, students will be able to write a Python class with instance methods and default parameters.

Writing a Dog class

Let’s define a Dog class to create new dogs from:

class Dog():
    def __init__(self, name, age=0):
        self.name = name
        self.age = age

    def bark(self):
        print(f'{self.name} says woof!')

💡 The naming convention for Python classes is UpperCamelCasing - just like in JavaScript.

__init__()

When a new dog is created, Python automatically calls the __init__() method.

In Python, methods with names beginning and ending with double underscores __ are known as dunder (double underscore) or magic methods. These are special methods that Python calls internally for certain operations. When referring to the __init__() method by name in conversation, it would be common to use the language dunder init.

__init__ is short for initialize, which is an appropriate name for a method used to initialize the new object’s properties.

❓ What method in JavaScript classes performs the same thing?

Inside the __init__() method, we define the attributes that exist on instances of the class. Instances of the Dog class will have name and age attributes.

The age = 0 in __init__’s parameter list is a default parameter. If we don’t give a dog an age when we create it by passing an argument in that position, then the dog’s age will be 0 as specified by = 0.

bark() is an instance method in this Dog class.

📚 An instance method is a function defined inside a class that operates on instances of that class. Put another way, every object created from the Dog class will have a bark() method!

self

You might recall using the this keyword in JavaScript. Every object-oriented programming language has a similar mechanism. This mechanism allows a method within an object to:

• Access other properties or methods in the same object.
• Use a single copy of a method in memory to interact with any number of object instances.

In Python, self functions similarly but with a key difference - self is not a keyword. Instead it’s a parameter name, and it is called self by convention.

When defining methods on Python classes, such as __init__() or bark(), the first parameter is typically named self. This parameter refers to the instance on which the method is being called.