Python Open–closed principle

Summary: in this tutorial, you’ll learn about the open-closed principle to extend the system without directly modifying existing code.

Introduction to the open-closed principle

The open-closed principle is one of the five principles in the SOLID principle. The letter O in the SOLID stands for the open-closed principle.

The open-closed principle states that a class, method, and function should be open for extension but closed for modification.

The open-closed principle sounds contradictory.

The purpose of the open-closed principle is to make it easy to add new features (or use cases) to the system without directly modifying the existing code.

Consider the following example:

class Person: def __init__(self, name): self.name = name def __repr__(self): return f'Person(name={self.name})' class PersonStorage: def save_to_database(self, person): print(f'Save the {person} to database') def save_to_json(self, person): print(f'Save the {person} to a JSON file') if __name__ == '__main__': person = Person('John Doe') storage = PersonStorage() storage.save_to_database(person)
Code language: Python (python)

In this example, the PersonStorage class has two methods:

  • The save_to_database() method saves a person to the database.
  • The save_to_json() method saves a person to a JSON file.

Later, if you want to save the Person’s object into an XML file, you must modify the PersonStorage class. It means that the PersonStorage class is not open for extension but modification. Hence, it violates the open-closed principle.

The open-closed principle example

To make the PersonStorage class conforms with the open-closed principle; you need to design the classes so that when you need to save the Person’s object into a different file format, you don’t need to modify it.

See the following class diagram:

First, define the PersonStorage abstract class that contains the save() abstract method:

from abc import ABC, abstractmethod class PersonStorage(ABC): @abstractmethod def save(self, person): pass
Code language: Python (python)

Second, create two classes PersonDB and PersonJSON that save the Person object into the database and JSON file. These classes inherit from the PersonStorage class:

class PersonDB(PersonStorage): def save(self, person): print(f'Save the {person} to database') class PersonJSON(PersonStorage): def save(self, person): print(f'Save the {person} to a JSON file')
Code language: Python (python)

To save the Person object into an XML file, you can define a new class PersonXML that inherits from the PersonStorage class like this:

class PersonXML(PersonStorage): def save(self, person): print(f'Save the {person} to a JSON file')
Code language: Python (python)

And you can save the Person‘s object into an XML file using the PersonXML class:

if __name__ == '__main__': person = Person('John Doe') storage = PersonXML() storage.save(person)
Code language: Python (python)

Put it all together:

from abc import ABC, abstractmethod class Person: def __init__(self, name): self.name = name def __repr__(self): return f'Person(name={self.name})' class PersonStorage(ABC): @abstractmethod def save(self, person): pass class PersonDB(PersonStorage): def save(self, person): print(f'Save the {person} to database') class PersonJSON(PersonStorage): def save(self, person): print(f'Save the {person} to a JSON file') class PersonXML(PersonStorage): def save(self, person): print(f'Save the {person} to a XML file') if __name__ == '__main__': person = Person('John Doe') storage = PersonXML() storage.save(person)
Code language: Python (python)

Summary

  • The open-closed principle allows you to design the system so that it is open for extension but closed for modification.
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