Classes in Python follow a definition format similar to other object-oriented languages. The classes can have members that are either public or private. The inheritance model allows the programmer to subclass and override methods of the parent class.

Let’s take a look at a simple class-based example:

class Greeter:
    greeting = ""
    def __init__(self, message):
        self.greeting = message
    def greet(self):
        return "Hello, " + self.greeting

greeter = Greeter("world")

We declare a new class Greeter. This class has three members: a property called greeting, a constructor, and a method greet.

You’ll notice that in the class when we refer to one of the members of the class we prepend self. This denotes that it’s a member access.

In the last line we create an instance of the Greeter by assigning a variable with a class construction. This calls into the constructor we defined earlier, creating a new object with the Greeter shape, and running the constructor to initialize it.


In python, we can use common object-oriented patterns. Of course, one of the most fundamental patterns in class-based programming is being able to extend existing classes to create new ones using inheritance.

Let’s take a look at an example:

class Animal:
    name = ""
    def __init__(self, theName): = theName
    def move(self, distanceInMeters = 0):
        print( + " moved " + str(distanceInMeters) + "m.")

class Snake(Animal):
    def __init__(self, name):
        Animal.__init__(self, name)
    def move(self, distanceInMeters = 5):
        Animal.move(self, distanceInMeters)

class Horse(Animal):
    def __init__(self, name):
        Animal.__init__(self, name)
    def move(self, distanceInMeters = 45):
        Animal.move(self, distanceInMeters)

sam = Snake("Sammy the Python")
tom = Horse("Tommy the Palomino")


This example covers quite a few of the inheritance features in python that are common to other languages. Here we see the class definition uses the Animal parameter to create a subclass. You can see this where Horse and Snake subclass the base class Animal and gain access to its features.

Derived classes that contain constructor functions must call Animal.__init__() which will execute the constructor function on the base class.

The example also shows how to override methods in the base class with methods that are specialized for the subclass. Here both Snake and Horse create a move method that overrides the move from Animal, giving it functionality specific to each class. Note that even though tom is declared as an Animal, since its value is a Horse, when tom.move(34) calls the overriding method in Horse:

Sammy the Python moved 5m.
Tommy the Palomino moved 34m.

Public, private, and protected modifiers

Public by default

In our examples, we’ve been able to freely access the members that we declared throughout our programs. If you’re familiar with classes in other languages, you may have noticed in the above examples we haven’t had to use the word public to accomplish this; for instance, C# requires that each member be explicitly labeled public to be visible. In python, each member is public by default.

You may still mark a member public explicitly. We could have written the Animal class from the previous section in the following way:

class Animal:
    name = ""
    def __init__(self, theName): = theName
    def move(self, distanceInMeters):
        print(str.format("{0} moved {1}m.",, distanceInMeters))

Understanding private

When a member is prefixes with a double underscore __ it is private and cannot be accessed from outside of its containing class. For example:

class Animal:
    __name = ""
    def __init__(self, theName): = theName
    def move(self, distanceInMeters):
        print(str.format("{0} moved {1}m.",, distanceInMeters))

Animal("Cat").name # Error: 'name' is private

Python is a structural type system. When we compare two different types, regardless of where they came from, if the types of all members are compatible, then we say the types themselves are compatible.

However, when comparing types that have private and protected members, we treat these types differently. For two types to be considered compatible, if one of them has a private member, then the other must have a private member that originated in the same declaration. The same applies to protected members.

Let’s look at an example to better see how this plays out in practice:

class Animal:
    __name = ""
    def __init__(self, theName):
        self.__name = theName
class Rhino(Animal):
    def __init__(self):
        Animal.__init__(self, "Rhino")
class Employee:
    __name = ""
    def __init__(self, theName):
        self.__name = theName

animal = Animal("Goat")
rhino = Rhino()
employee = Employee("Bob")

animal = rhino
animal = employee # Error: 'Animal' and 'Employee' are not compatible

In this example, we have an Animal and a Rhino, with Rhino being a subclass of Animal. We also have a new class Employee that looks identical to Animal in terms of shape. We create some instances of these classes and then try to assign them to each other to see what will happen. Because Animal and Rhino share the private side of their shape from the same declaration of __ name = "" in Animal, they are compatible. However, this is not the case for Employee. When we try to assign from an Employee to Animal we get an error that these types are not compatible. Even though Employee also has a private member called name, it’s not the one we declared in Animal.

Understanding protected

The protected modifier _ acts much like the private modifier with the exception that members declared protected can also be accessed by instances of deriving classes. For example,

class Person:
    _name = ""
    def __init__(self, name): = name

class Employee(Person):
    __department = ""

    def __init__(self, name, department):
        Person.__init__(self, name)
        self.department = department

    def getElevatorPitch(self):
        return "Hello, my name is " + + " and I work in " + self.department + "."

howard = Employee("Howard", "Sales")
print( # error

Notice that while we can’t use name from outside of Person, we can still use it from within an instance method of Employee because Employee derives from Person.