Classes

A class is a data structure that contains attributes and methods. You have already used classes in Python. For example, int and list are both classes.

print(int) # <class 'int'>
print(list) # <class 'list'>

In the list class the attribute is the data stored in the list and the methods are functions that can be called on the list, such as append and sort.

A class instance is an object that is created from a class. Using int as an example, 1 and 5 would be instances of the int class.

We can use the built-in function isinstance to check if an object is an instance of a class. Similarly, we can use the built-in function type to get the class of an object.

isinstance(5, int) # True
isinstance(5, list) # False
print(type(5)) # <class 'int'>

Creating a class

The keyword class is used to create a class.

class ClassDefinition:
    attribute = 10
    def method():
        return 20

print(ClassDefinition.attribute) # 10
print(ClassDefinition.method()) # 20

Creating an instance

A class instance is an object that is created from a class. The self argument is automatically passed into instance methods when the method is called. It can be used to access the attributes and methods of the object.

class ClassDefinition:
    attribute = 10
    def method(self):
        return self.attribute * 2

# use the class to create an object
class_instance = ClassDefinition()
print(class_instance.attribute) # 10
print(class_instance.method()) # 20

The __init__ method

Dunder methods are special methods that are used to define the behavior of a class. The __init__ dunder method is one of these special methods. It is called when an object is created from a class to initialize the object with any attributes that are required.

class ClassDefinition:
    def __init__(self, attribute):
        self.attribute = attribute

    def method(self):
        return self.attribute * 2

# use the class to create an object
class_instance = ClassDefinition(20)
print(class_instance.attribute) # 20
print(class_instance.method()) # 40

Exercise

1. Create a class called Person
2. Add two attributes: name and age that can be set when the object is created.
3. Create a method called introduce that prints a string in the format:

Hi, My name is Joe and I am 12 years old.