2  Dependency Injection

Dependency Injection (DI) is a design pattern in software engineering that deals with how objects acquire their dependencies (i.e., other objects or resources they need to function).

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2.0.1 🧠 Concept Overview

• “Dependency”: An object or service that another object depends on.
• “Injection”: Providing that dependency from outside rather than letting the object create it by itself.

Instead of a class instantiating its dependencies (tight coupling), they are passed in (injected) from the outside (loose coupling).

This helps improve:

• Modularity
• Testability
• Flexibility

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2.0.2 🔧 Types of Dependency Injection

Type	How it’s Injected
Constructor Injection	Via constructor parameters
Setter Injection	Via setter methods or public properties
Interface Injection	The dependency provides an injector interface

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2.0.3 🧪 Example in Python (Constructor Injection)

Let’s say we have a DatabaseService that’s used in a UserRepository:

2.0.3.1 🔴 Without Dependency Injection (Tightly Coupled)

class UserRepository:
    def __init__(self):
        self.db = DatabaseService()  # hard-coded dependency

    def get_user(self, user_id):
        return self.db.query(f"SELECT * FROM users WHERE id = {user_id}")

• UserRepository creates the DatabaseService → hard to test or reuse with another DB.

2.0.3.2 🟢 With Dependency Injection (Loosely Coupled)

class UserRepository:
    def __init__(self, db_service):
        self.db = db_service  # dependency injected from outside

    def get_user(self, user_id):
        return self.db.query(f"SELECT * FROM users WHERE id = {user_id}")

Now you can pass in any db_service:

# Example concrete dependency
class DatabaseService:
    def query(self, sql):
        print(f"Executing SQL: {sql}")

# Use dependency injection
db = DatabaseService()
repo = UserRepository(db)
repo.get_user(123)

You can easily mock or replace the DatabaseService in testing:

# A mock dependency
class MockDatabaseService:
    def query(self, sql):
        return {"id": 123, "name": "Test User"}

mock_repo = UserRepository(MockDatabaseService())
print(mock_repo.get_user(123))  # → {'id': 123, 'name': 'Test User'}

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2.0.4 🧩 Summary Table

Feature	Without DI	With DI
Dependency creation	Inside the class	Outside the class
Coupling	Tight	Loose
Testability	Hard (need real objects)	Easy (mock dependencies)
Flexibility	Low	High

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