Dependency Injection

What is Dependency Injection?

Dependency Injection (DI) is a design pattern where an object receives its dependencies from an external source instead of creating them itself. A dependency is any object another object requires to perform its work. Examples:

Database
Repository
Logger
Email Service
Configuration

Without Dependency Injection

The object creates its own dependency.

class Engine:
    pass

class Car:
    def __init__(self):
        self.engine = Engine()      # Tight coupling

Problems

Tight coupling
Hard to test
Difficult to replace dependencies

With Dependency Injection

The dependency is supplied from outside.

class Engine:
    pass

class Car:
    def __init__(self, engine):
        self.engine = engine

Usage:

engine = Engine()
car = Car(engine)

Now Car only uses the engine—it doesn’t create it.

Real-world Example: Repository & Service

Without DI

class StudentRepository:
    def get_all(self):
        return ["Alice", "Bob"]


class StudentService:
    def __init__(self):
        self.repo = StudentRepository()

    def list_students(self):
        return self.repo.get_all()

With DI

class StudentRepository:
    def get_all(self):
        return ["Alice", "Bob"]


class StudentService:
    def __init__(self, repository):
        self.repo = repository

    def list_students(self):
        return self.repo.get_all()

Usage:

repo = StudentRepository()
service = StudentService(repo)

print(service.list_students())

Later, we can replace the repository without changing the service.

service = StudentService(MockStudentRepository())

Benefits

Loose coupling
Easier testing
Better maintainability
Easily replace implementations
More reusable code

Types of Dependency Injection

Constructor Injection (Most Common)

class UserService:
    def __init__(self, database):
        self.database = database

Setter Injection

service.set_database(database)

Method Injection

service.get_user(database)

FastAPI Example

FastAPI automatically performs dependency injection.

from fastapi import Depends

def get_repository():
    return StudentRepository()

@app.get("/students")
def get_students(
    repo = Depends(get_repository)
):
    return repo.get_all()

A more realistic example using a service:

def get_repository():
    return StudentRepository()

def get_service(
    repo = Depends(get_repository)
):
    return StudentService(repo)

@app.get("/students")
def get_students(
    service = Depends(get_service)
):
    return service.list_students()

Summary

Dependency Injection is a technique where:

Objects receive their dependencies instead of creating them.
Components remain loosely coupled.
Dependencies can be replaced easily.
Testing becomes much simpler.
Frameworks like FastAPI, Spring Boot, and ASP.NET Core automate dependency injection.

Python Foundations

Python Intermediate

Python Advanced

What is Dependency Injection?

Without Dependency Injection

With Dependency Injection

Real-world Example: Repository & Service

Without DI

With DI

Benefits

Types of Dependency Injection

Constructor Injection (Most Common)

Setter Injection

Method Injection

FastAPI Example

Summary

​What is Dependency Injection?

​Without Dependency Injection

​With Dependency Injection

​Real-world Example: Repository & Service

​Without DI

​With DI

​Benefits

​Types of Dependency Injection

​Constructor Injection (Most Common)

​Setter Injection

​Method Injection

​FastAPI Example

​Summary

What is Dependency Injection?

Without Dependency Injection

With Dependency Injection

Real-world Example: Repository & Service

Without DI

With DI

Benefits

Types of Dependency Injection

Constructor Injection (Most Common)

Setter Injection

Method Injection

FastAPI Example

Summary