Creating Classes

Creating Classes

When we create a class we are essentially creating our own data type. To do this we must first come up with a name for our class and populate it with some methods.

class Dog():
    def __init__(self):
        pass

    def speak(self):
        pass

The name of this class is "Dog" and currently it has two methods: init and speak.

Typically when we create a class we create a method that is known as a constructor. A constructor is what will be called automatically when we create a new instance of that class. To create a constructor we must use the method name init(). We do not need to call this method by doing something like "instance.init()" because when we first create a Dog object it will be automatically called.

To create a new instance of our Dog class we can do the following:

class Dog():
    def __init__(self):
        print("Created a dog")

    def speak(self):
        pass

tim = Dog()  # This will print "Created a dog" because __init__ will automatically be called

The Self Keyword

You may have noticed that each of my methods above contain the keyword self as a parameter. For now all of the methods we make must do just this. When we call methods on an instance of the class the name of that instance is automatically passed to the method as the argument self. This allows us to access and change attributes that are specific to this instance.

To create a new attribute we must use the self keyword in the following way.

class Dog():
    def __init__(self, name):
        self.name = name  # self.name is now an attribute of the Dog class and has a specific value for each instance

    def speak(self):
        pass

tim = Dog("Tim") 

Since the init methods now takes two arguments (self and name) we must pass a name when we create a new Dog object.

Since this name is specific to each instance when we create multiple Dog objects they all may have different names.

class Dog():
    def __init__(self, name):
        self.name = name  # self.name is now an attribute of the Dog class and has a specific value for each instance

    def speak(self):
        print(self.name)

tim = Dog("Tim") 
dog2 = Dog("Fred")

tim.speak()  # Prints "Tim"
dog2.speak() # Prints "Fred"

Changing Attributes

Now that we know how to create attributes it's time to talk about accessing and changing them. We can change attributes in two ways. It is best practice to change an attribute within the class like so.

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

    def speak(self):
        print("I am", self.name, "and I am", self.age, "years old")

    def change_age(self, age)
        self.age = age  # This changes the age attribute

tim = Dog("Tim", 5)
tim.change_age(7)
tim.speak() # This will print "I am Tim and I am 7 years old" 

The second way to change attribute is to do so outside of the class, like so:

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

    def speak(self):
        print("I am", self.name, "and I am", self.age, "years old")


tim = Dog("Tim", 5)
tim.age = 7
tim.speak() # This will print "I am Tim and I am 7 years old" 

Note: We can add new attributes to a class from outside of the init method. However, we must do this within another method in the class. (Watch video if confused)