Why dependency injection matters in FastAPI - See It in Action

Why dependency injection matters

📖 Scenario: You are building a simple FastAPI web service that returns user information. To keep your code clean and easy to test, you will use dependency injection to provide the user data source.

🎯 Goal: Create a FastAPI app that uses dependency injection to supply a user data dictionary to an endpoint. This will show how dependency injection helps separate concerns and makes testing easier.

📋 What You'll Learn

Create a dictionary called users with exact entries: 1: 'Alice', 2: 'Bob', 3: 'Charlie'

Create a dependency function called get_users that returns the users dictionary

Create a FastAPI app instance called app

Create a GET endpoint /user/{user_id} that uses get_users as a dependency and returns the user name for the given user_id

Use dependency injection with Depends(get_users) in the endpoint function parameter

💡 Why This Matters

🌍 Real World

Dependency injection is used in real web services to keep code modular and testable. It helps separate how data is provided from how it is used.

💼 Career

Understanding dependency injection is important for backend developers working with FastAPI or similar frameworks to write clean, maintainable, and testable code.

1

Create the user data dictionary

Create a dictionary called users with these exact entries: 1: 'Alice', 2: 'Bob', 3: 'Charlie'

FastAPI

# Create the users dictionary with IDs and names
# Your code here

Need a hint?

Use curly braces to create a dictionary with keys 1, 2, 3 and values 'Alice', 'Bob', 'Charlie'.

2

Create the dependency function

Create a function called get_users that returns the users dictionary

FastAPI

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

# Define get_users function that returns users
# Your code here

Need a hint?

Define a function named get_users that simply returns the users dictionary.

3

Create the FastAPI app and import Depends

Import FastAPI and Depends from fastapi and create a FastAPI app instance called app

FastAPI

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

def get_users():
    return users

# Import FastAPI and Depends, then create app instance
# Your code here

Need a hint?

Use from fastapi import FastAPI, Depends and then app = FastAPI().

4

Create the GET endpoint using dependency injection

Create a GET endpoint /user/{user_id} in app that uses get_users as a dependency with Depends(get_users) and returns the user name for the given user_id

FastAPI

from fastapi import FastAPI, Depends

users = {1: 'Alice', 2: 'Bob', 3: 'Charlie'}

def get_users():
    return users

app = FastAPI()

# Create GET endpoint /user/{user_id} using Depends(get_users)
# Your code here

Need a hint?

Use @app.get('/user/{user_id}') decorator and a function with parameters user_id: int and users: dict = Depends(get_users). Return a dictionary with the user name or 'Unknown' if not found.