Dependency Inversion Principle (DIP)

“High-level modules should not depend on low-level modules; both should depend on abstractions.” In short, depend on abstractions, not on concrete implementations.

Purpose

Decouples higher-level logic from low-level details, allowing those details to change without affecting high-level code.
Improves maintainability and testability – you can swap out or mock lower-level components (e.g., for unit tests or new requirements) without rewriting the core business logic.
Encourages layering and reuse: common abstractions can be defined and different implementations provided (for example, different database backends, different notification methods, etc.), all interchangeable from the perspective of the high-level code.

Minimal Example

In the first part below, ReportGenerator directly creates and uses a concrete EmailSender. This tight coupling means if we wanted to use a different notification method or change EmailSender, we’d have to modify ReportGenerator (a high-level module depending on a low-level one). The DIP-compliant solution introduces an abstract Notifier interface that ReportGenerator relies on. Concrete implementations like EmailNotifier and SMSNotifier can be injected into ReportGenerator. Now ReportGenerator depends only on the Notifier abstraction, not on any specific email/SMS class.

class EmailSender:
    def send_email(self, message):
        print(f"Email: {message}")

class ReportGenerator:
    def __init__(self):
        self.email_sender = EmailSender()   # Direct dependency on a concrete class
    def generate_report(self):
        # ... generate report ...
        self.email_sender.send_email("Report ready.")  # hard-coded email notification

# Applying DIP: introduce an abstraction for notifications
class Notifier:
    def send(self, message):
        raise NotImplementedError

class EmailNotifier(Notifier):
    def send(self, message):
        print(f"Email: {message}")

class SMSNotifier(Notifier):
    def send(self, message):
        print(f"SMS: {message}")

class ReportGeneratorDIP:
    def __init__(self, notifier: Notifier):
        self.notifier = notifier            # Depend on abstraction
    def generate_report(self):
        # ... generate report ...
        self.notifier.send("Report ready.")  # Use the provided notifier, not knowing its type

# Usage:
rg = ReportGeneratorDIP(EmailNotifier())
rg.generate_report()  # Uses email under the hood

rg = ReportGeneratorDIP(SMSNotifier())
rg.generate_report()  # Uses SMS under the hood

# The high-level ReportGeneratorDIP is unaware of how notifications are sent.
# We could even create a MockNotifier for testing or add new Notifier types without changing ReportGeneratorDIP.

More Realistic Example

Consider an order processing system with multiple payment providers. Without DIP, the OrderService might directly instantiate a specific payment processor (say, Stripe), making it hard to switch to another provider. Using DIP, we define an abstract PaymentProcessor interface. OrderService depends on this abstraction and is provided an implementation (via constructor). This way, OrderService can work with any payment processor (Stripe, PayPal, etc.) interchangeably, and adding a new one doesn’t require altering OrderService’s code.

class PaymentProcessor:
    def process_payment(self, order):
        raise NotImplementedError

class StripeProcessor(PaymentProcessor):
    def process_payment(self, order):
        print(f"Processing order {order['id']} via Stripe...")

class PayPalProcessor(PaymentProcessor):
    def process_payment(self, order):
        print(f"Processing order {order['id']} via PayPal...")

class OrderService:
    def __init__(self, processor: PaymentProcessor):
        self.processor = processor  # depends on abstract PaymentProcessor
    def place_order(self, order):
        # ... validate and prepare order ...
        self.processor.process_payment(order)   # delegate payment to the injected processor
        print("Order placed successfully.")

# Usage: injecting different payment processors without changing OrderService
order = {"id": 101, "amount": 250}
service = OrderService(StripeProcessor())
service.place_order(order)
# Output: Processing order 101 via Stripe...
#         Order placed successfully.

service = OrderService(PayPalProcessor())
service.place_order(order)
# Output: Processing order 101 via PayPal...
#         Order placed successfully.

# OrderService is oblivious to which processor is used – we could add a new processor (e.g., ApplePayProcessor)
# and use it here without modifying the OrderService code at all.

Fundamentals