Python Single Responsibility Principle

Summary: in this tutorial, you’ll learn about the single responsibility principle and how to implement it in Python.

What is SOLID

SOLID is an abbreviation that stands for five software design principles compiled by Uncle Bob:

The single responsibility is the first principle in the SOLID principles.

Introduction to the single responsibility principle

The single responsibility principle (SRP) states that every class, method, and function should have only one job or one reason to change.

The purposes of the single responsibility principle are to:

Create high cohesive and robust classes, methods, and functions.
Promote class composition
Avoid code duplication

Let’s take a look at the following Person class:

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

    def __repr__(self):
        return f'Person(name={self.name})'

    @classmethod
    def save(cls, person):
        print(f'Save the {person} to the database')


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

This Person class has two jobs:

Manage the person’s property.
Store the person in the database.

Later, if you want to save the Person into different storage such as a file, you’ll need to change the save() method, which also changes the whole Person class.

To make the Person class conforms to the single responsibility principle, you’ll need to create another class that is in charge of storing the Person to a database. For example:

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

    def __repr__(self):
        return f'Person(name={self.name})'


class PersonDB:
    def save(self, person):
        print(f'Save the {person} to the database')


if __name__ == '__main__':
    p = Person('John Doe')

    db = PersonDB()
    db.save(p)Code language: Python (python)

In this design, we separate the Person class into two classes: Person and PersonDB:

The Person class is responsible for managing the person’s properties.
The PersonDB class is responsible for storing the person in the database.

In this design, if you want to save the Person to different storage, you can define another class to do that. And you don’t need to change the Person class.

When designing classes, you should put related methods that have the same reason for change together. In other words, you should separate classes if they change for different reasons.

This design has one issue that you need to deal with two classes: Person and PersonDB.

To make it more convenient, you can use the facade pattern so that the Person class will be the facade for the PersonDB class like this:

class PersonDB:
    def save(self, person):
        print(f'Save the {person} to the database')


class Person:
    def __init__(self, name):
        self.name = name
        self.db = PersonDB()

    def __repr__(self):
        return f'Person(name={self.name})'

    def save(self):
        self.db.save(person=self)


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

Summary

The single responsibility principle (SRP) states that every class, method, or function should have only one job or one reason to change.
Use the single responsibility principle to separate classes, methods, and functions with the same reason for changes.

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