What is Single Responsibility principle?

The Single Responsibility Principle (SRP) — Simple but Powerful

Starting from this post, I’ll be breaking down each of the SOLID principles one by one. Let’s begin with the first one — the Single Responsibility Principle (SRP).

SRP says:

A class should have only one reason to change. — Robert C. Martin

In simpler terms: a class should do just one job. When a class starts doing too many things at once, it becomes harder to test, harder to extend, and more fragile when you modify it.

A Class Doing Too Much

Here’s a common anti-pattern: one class fetching data, processing it, saving it — and also handling multiple export formats:

class InventoryHandler:
    def fetch_data(self): ...
    def process_data(self): ...
    def save_data(self): ...
 
    def export_csv(self): ...
    def export_xlsx(self): ...
 
    def export(self, format: str):
        if format == "csv":
            return self.export_csv()
        elif format == "xlsx":
            return self.export_xlsx()
 
    def run(self):
        self.fetch_data()
        self.process_data()
        self.save_data()
 

At first glance it may look fine — everything is grouped in one place. But the moment you want to add a new export format (like PDF), you’re forced to change this class. Even though the data fetching logic didn’t change, this class still has to be modified. That means multiple reasons to change, which violates SRP.

Refactoring with SRP in Mind

To apply SRP, we separate the responsibilities:

• One class deals with data retrieval and persistence.
• Another class handles report formatting and exporting.

class InventoryHandler:
    def fetch_data(self): ...
    def process_data(self): ...
    def save_data(self): ...
 
    def run(self):
        self.fetch_data()
        self.process_data()
        self.save_data()
 
class ReportHandler:
    def __init__(self, data) -> None:
        self.data = data
 
    def export_csv(self): ...
    def export_xlsx(self): ...
 
    def export(self, format: str):
        if format == "csv":
            return self.export_csv()
        elif format == "xlsx":
            return self.export_xlsx()
 

Now, if you want to add export_pdf() one day, you only modify ReportHandler — not the business logic in InventoryHandler. Each class has a single reason to change, which is exactly what SRP aims for.

Single Responsibility Principle is often misunderstood as “a class should do very little.” In reality, it simply means a class should have only one reason to change. By separating concerns, your code becomes easier to modify, easier to test, and much more maintainable in the long term.

In the next post, I’ll build on top of this design to show how SRP naturally leads into the second SOLID principle — the Open/Closed Principle.