Python - Interfaces

In software engineering, an interface is a software architectural pattern. An interface is like a class but its methods just have prototype signature definition without any body to implement. The recommended functionality needs to be implemented by a concrete class.

In languages like Java, there is interface keyword which makes it easy to define an interface. Python doesn't have it or any similar keyword. Hence the same ABC class and @abstractmethod decorator is used as done in an abstract class.

An abstract class and interface appear similar in Python. The only difference in two is that the abstract class may have some non-abstract methods, while all methods in interface must be abstract, and the implementing class must override all the abstract methods.

Example

from abc import ABC, abstractmethod
class demoInterface(ABC):
   @abstractmethod
   def method1(self):
      print ("Abstract method1")
      return

   @abstractmethod
   def method2(self):
      print ("Abstract method1")
      return

The above interface has two abstract methods. As in abstract class, we cannot instantiate an interface.

   obj = demoInterface()
         ^^^^^^^^^^^^^^^
TypeError: Can't instantiate abstract class demoInterface with abstract methods method1, method2

Let us provide a class that implements both the abstract methods. If doesn't contain implementations of all abstract methods, Python shows following error −

   obj = concreteclass()
         ^^^^^^^^^^^^^^^
TypeError: Can't instantiate abstract class concreteclass with abstract method method2

The following class implements both methods −

class concreteclass(demoInterface):
   def method1(self):
      print ("This is method1")
      return
   
   def method2(self):
      print ("This is method2")
      return
      
obj = concreteclass()
obj.method1()
obj.method2()

Output

When you execute this code, it will produce the following output −

This is method1
This is method2