In this lesson, you’ll learn where classes and objects are used in real software, as well as how they’re defined in Python.

We can define an empty Dog class like this:

Python
      
    
class Dog:
    pass

Classes contain characteristics called Attributes. We make a distinction between instance attributes and class attributes.

Instance Attributes are unique to each object, (an instance is another name for an object). Here, any Dog object we create will be able to store its name and age. We can change either attribute of either dog, without affecting any other dog objects we’ve created:

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class Dog:

    def __init__(self, name, age):
        self.name = name
        self.age = age

This __init__ is called the initializer. It is automatically called when we instantiate the class. It’s job is to make sure the class has any attributes it needs. It’s sometimes also used to make sure that the object is in a valid state when it’s instantiated, like making sure the user didn’t enter a negative age for the dog.

We have to include the self parameter so that our initializer has a reference to the new object being instantiated.

Class Attributes are unique to each class. Each instance of the class will have this attribute. It’s sometimes used to specify a defualt value that all objects should have after they’ve been instantiated. Here, our class attribute is species:

Python
      
    
class Dog:

    species = 'mammal'

    def __init__(self, name, age):
        self.name = name
        self.age = age

Class and Instance Attributes