The Simplest Way to get Started with Python Classes

One of the basic building blocks of Python is classes. We’ve already covered the basics of functions and classes, this post will be more of a quickstart with classes in Python. For the video version:

Creating a Simple Class in Python

Alright let’s get started – the only thing you need to know about classes to get started is that they’re just custom objects. They have properties, a constructor/initializer, and methods. Let’s take a look at a simple Shape class.

class Shape:
    # declare a class property
    dimensions = 2

A class is not required to have properties and methods, simply declaring a property makes this a valid class. However, this is pretty useless, so let’s add a constructor/initializer method to our class. Constructors in Python are declared with the keyword __init__ (now you know why I call them initializers too). For our Shape constructor we’ll pass in one parameter – a number of sides. All we’ll do here is set self.sides – the sides property of the Shape – to the passed in sides variable.

class Shape:
    # declare a class property
    dimensions = 2
    # declare a constructor/initializer
    def __init__(self, sides):
        self.sides = sides

Now when we create a Shape we’ll need to declare the number of sides it has. Our Shape object is still missing something, let’s give it another method. We’ll have our new method validate our Shape. All we need to do here is check that our Shape has more than two sides.

class Shape:
    # declare a class property
    dimensions = 2
    # declare a constructor/initializer
    def __init__(self, sides):
        self.sides = sides
        self.validator()
    # declare a class function
    def validator(self):
        assert self.sides > 2

Now when we try to create a Shape object with two sides we should get an Assertion Error like the image below.

Extending a Class in Python (Subclasses)

Okay now check this out, this next part is cool. You can “extend” classes into subclasses and they will “inherit” properties from the super class. In our example of Shapes and Triangles, Triangle is a subclass of Shape. Triangles have access to all the same properties and methods as Shape but also have additional properties and/or methods. 

We’ll make a Triangle class that extends a Shape with a constructor that takes three parameters for the side lengths. In our constructor, we’ll call super() to call the constructor for Shape with three sides and set our self variables equal to the passed in parameters. We’ll also have a validate function in Triangle. This time our function is called __validate, pay attention because this is important – the double underscore in front of the function name indicates this is a private function. This means it can only be accessed from inside the class. Our Triangle validate function will check that the passed in sides form a valid triangle.

class Triangle(Shape):
    # triangle constructor
    def __init__(self, a, b, c):
        super().__init__(3)
        self.a = a
        self.b = b
        self.c = c
        self.__validate()
   
    # private function to class
    def __validate(self):
        assert self.a + self.b > self.c
        assert self.a + self.c > self.b
        assert self.b + self.c > self.a

If we try to call Triangle to create a degenerate Triangle like one with sides of length 1, 2, and 3, we should get an assertion error like the image below.

We should be able to create a Triangle with sides of length 2, 2, and 3 though. 

Now let’s also try to call __validate just to validate (pun intended) that it’s a private function. We should see an Attribute Error like the image below.

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