Introduction

When building a payment processing system, it’s essential to establish a consistent structure while allowing flexibility for different payment gateways like PayPal, Stripe, or UPI. You might have a class named PaymentProcessor to define the common interface for all payment methods — but it should never be instantiated directly. Moreover, every subclass must implement its own version of the critical process_payment() method. So, how can we enforce this design in Python?

Leveraging Abstract Base Classes (ABCs)
Python provides a powerful tool to achieve this: Abstract Base Classes from the abc module.

Here’s how you can structure your code:

 from abc import ABC, abstractmethod

class PaymentProcessor(ABC):
    
    @abstractmethod
    def process_payment(self, amount):
        pass

If a subclass fails to implement this method, it will raise a TypeError when you try to instantiate it.

PaymentProcessor inherits from ABC, making it an abstract base class.

The @abstractmethod decorator ensures that process_payment() must be implemented by all subclasses.

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Now, create concrete subclasses like this:

class PayPalProcessor(PaymentProcessor):
    
    def process_payment(self, amount):
        print(f"Processing ₹{amount} via PayPal.")

class StripeProcessor(PaymentProcessor):
    
    def process_payment(self, amount):
        print(f"Processing ₹{amount} via Stripe.")

Trying to instantiate PaymentProcessor directly will result in an error:

processor = PaymentProcessor()  # ❌ This will raise TypeError

Likewise, any subclass that doesn’t implement process_payment() cannot be instantiated either.

Conclusion

By using abstract base classes, you ensure consistency across your payment processing system while preventing misuse of the base class. The PaymentProcessor acts as a blueprint, and every concrete subclass is held accountable to implement its own version of process_payment(). This design makes your code clean, predictable, and easy to maintain — a vital aspect when dealing with critical tasks like handling user payments.