Sub-dependencies in FastAPI - Mini Project: Build & Apply

Using Sub-dependencies in FastAPI

📖 Scenario: You are building a simple FastAPI app that needs to check user authentication and then get user details. To keep your code clean, you want to use sub-dependencies: one dependency to check authentication, and another that depends on it to get user info.

🎯 Goal: Build a FastAPI app with two dependencies: get_current_user that depends on verify_token. The app should have one route /profile that returns the current user's name.

📋 What You'll Learn

Create a dependency function called verify_token that returns a fixed token string.

Create a sub-dependency function called get_current_user that depends on verify_token and returns a dictionary with a name key.

Create a FastAPI app instance called app.

Create a GET route /profile that uses get_current_user as a dependency and returns the user's name.

💡 Why This Matters

🌍 Real World

In real apps, authentication and user info fetching are often done with dependencies. Sub-dependencies help organize these steps cleanly.

💼 Career

Understanding FastAPI dependencies and sub-dependencies is essential for backend developers building secure and maintainable APIs.

1

Create the verify_token dependency

Create a function called verify_token that returns the string 'secrettoken123'.

FastAPI

# Your code here

Need a hint?

This function simulates checking a token and returns a fixed string.

2

Create the get_current_user sub-dependency

Create a function called get_current_user that takes a parameter token with a dependency on verify_token using Depends. It should return a dictionary {'name': 'Alice'}.

FastAPI

def verify_token():
    return 'secrettoken123'

from fastapi import Depends

# Your code here

Need a hint?

Use Depends(verify_token) to declare the sub-dependency.

3

Create the FastAPI app instance

Import FastAPI and create an app instance called app.

FastAPI

def verify_token():
    return 'secrettoken123'

from fastapi import Depends

def get_current_user(token: str = Depends(verify_token)):
    return {'name': 'Alice'}

# Your code here

Need a hint?

Remember to import FastAPI and create app = FastAPI().

4

Create the /profile route using the sub-dependency

Create a GET route /profile on app that uses get_current_user as a dependency with Depends. The route function should accept a parameter user and return {'user_name': user['name']}.

FastAPI

from fastapi import FastAPI, Depends

def verify_token():
    return 'secrettoken123'

from fastapi import Depends

def get_current_user(token: str = Depends(verify_token)):
    return {'name': 'Alice'}

app = FastAPI()

# Your code here

Need a hint?

Use @app.get('/profile') and Depends(get_current_user) to inject the user.