Router prefix and tags in FastAPI - Mini Project: Build & Apply

FastAPI Router Prefix and Tags

📖 Scenario: You are building a simple web API for a bookstore. You want to organize your routes using FastAPI routers. This helps keep your code clean and easy to manage.

🎯 Goal: Create a FastAPI router with a prefix and tags to group book-related endpoints. This will help users and developers understand the API structure better.

📋 What You'll Learn

Create a FastAPI router named book_router.

Set the router prefix to /books.

Add the tag Books to the router.

Define a GET endpoint / inside the router that returns a list of book titles.

Include the router in the main FastAPI app.

💡 Why This Matters

🌍 Real World

Organizing API routes with routers is common in real-world web applications to keep code clean and maintainable.

💼 Career

Understanding FastAPI routers, prefixes, and tags is essential for backend developers building scalable APIs.

1

Create the initial FastAPI app and router

Import FastAPI and APIRouter from fastapi. Create a FastAPI app called app and a router called book_router.

FastAPI

from fastapi import FastAPI, APIRouter

# Create the FastAPI app and router
# Your code here

Hint

Use app = FastAPI() to create the app and book_router = APIRouter() to create the router.

2

Add prefix and tags to the router

Modify the book_router to have the prefix /books and the tags list containing Books.

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()
book_router = APIRouter()
# Add prefix '/books' and tags ['Books'] to book_router
# Your code here

Hint

Pass prefix='/books' and tags=['Books'] as arguments when creating book_router.

3

Add a GET endpoint to the router

Inside book_router, define a GET endpoint at / that returns the list ["The Hobbit", "1984", "Python 101"]. Use the function name get_books.

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()
book_router = APIRouter(prefix='/books', tags=['Books'])

# Define GET endpoint '/' inside book_router
# Your code here

Hint

Use @book_router.get('/') decorator and define def get_books(): that returns the list.

4

Include the router in the main app

Add the book_router to the main app using app.include_router(book_router).

FastAPI

from fastapi import FastAPI, APIRouter

app = FastAPI()
book_router = APIRouter(prefix='/books', tags=['Books'])

@book_router.get('/')
def get_books():
    return ["The Hobbit", "1984", "Python 101"]

# Include book_router in app
# Your code here

Hint

Use app.include_router(book_router) to add the router to the app.

Practice

1. What is the main purpose of using a prefix in a FastAPI router?

easy

A. To add authentication to routes automatically

B. To group routes under a common URL path segment

C. To change the HTTP method of all routes

D. To disable routes temporarily

Solution

1. Step 1: Understand what prefix does in FastAPI routers

   The prefix adds a common path segment to all routes defined in that router, grouping them logically.

2. Step 2: Compare options with prefix purpose

   Only To group routes under a common URL path segment correctly describes grouping routes under a shared URL path segment.

3. Final Answer:

   To group routes under a common URL path segment -> Option B

4. Quick Check:

   Router prefix = group routes [OK]

Hint: Prefix groups routes by URL path segment [OK]

Common Mistakes:

• Thinking prefix changes HTTP methods
• Confusing prefix with authentication
• Assuming prefix disables routes

2. Which of the following is the correct way to add a prefix and tags to a FastAPI router?

easy

A. router = APIRouter(prefix='/items', tags=['items'])

B. router = APIRouter(tags='/items', prefix=['items'])

C. router = APIRouter(prefix=['items'], tags='/items')

D. router = APIRouter(prefix='/items', tags='/items')

Solution

1. Step 1: Recall correct parameter types for prefix and tags

   Prefix expects a string starting with '/', tags expects a list of strings.

2. Step 2: Check each option's syntax

   router = APIRouter(prefix='/items', tags=['items']) uses prefix as string and tags as list correctly; others mix types incorrectly.

3. Final Answer:

   router = APIRouter(prefix='/items', tags=['items']) -> Option A

4. Quick Check:

   Prefix string + tags list = correct syntax [OK]

Hint: Prefix is string, tags is list of strings [OK]

Common Mistakes:

• Using list for prefix instead of string
• Using string for tags instead of list
• Mixing order of parameters incorrectly

3. Given this code snippet, what is the full path of the endpoint?

from fastapi import APIRouter

router = APIRouter(prefix='/users', tags=['users'])

@router.get('/profile')
async def get_profile():
    return {'msg': 'User profile'}

medium

A. /profile/users

B. /profile

C. /users

D. /users/profile

Solution

1. Step 1: Identify router prefix and route path

   The router has prefix '/users' and the route path is '/profile'.

2. Step 2: Combine prefix and route path

   Full path is prefix + route path = '/users' + '/profile' = '/users/profile'.

3. Final Answer:

   /users/profile -> Option D

4. Quick Check:

   Prefix + route path = full endpoint path [OK]

Hint: Full path = prefix + route path [OK]

Common Mistakes:

• Ignoring prefix and using only route path
• Swapping prefix and route path order
• Confusing tags with path

4. What is wrong with this FastAPI router code?

router = APIRouter(prefix=['/items'], tags='items')

medium

A. prefix should be a list, tags should be a string

B. Both prefix and tags should be strings

C. prefix should be a string, tags should be a list of strings

D. No error, this is valid syntax

Solution

1. Step 1: Check expected types for prefix and tags

   Prefix must be a string starting with '/', tags must be a list of strings.

2. Step 2: Analyze given code

   Prefix is given as a list, tags as a string, both reversed from expected types.

3. Final Answer:

   prefix should be a string, tags should be a list of strings -> Option C

4. Quick Check:

   Prefix = string, tags = list [OK]

Hint: Prefix string, tags list; reverse causes error [OK]

Common Mistakes:

• Using list for prefix
• Using string for tags
• Assuming both can be strings

5. You want to create two routers in FastAPI: one for /products tagged 'products' and another for /orders tagged 'orders'. How do you correctly set prefixes and tags to keep your API organized?

hard

A. router_products = APIRouter(prefix='/products', tags=['products'])\nrouter_orders = APIRouter(prefix='/orders', tags=['orders'])

B. router_products = APIRouter(prefix=['products'], tags='products')\nrouter_orders = APIRouter(prefix=['orders'], tags='orders')

C. router_products = APIRouter(prefix='/products', tags='products')\nrouter_orders = APIRouter(prefix='/orders', tags='orders')

D. router_products = APIRouter(tags=['products'])\nrouter_orders = APIRouter(prefix='/orders')

Solution

1. Step 1: Recall correct types for prefix and tags

   Prefix must be a string starting with '/', tags must be a list of strings.

2. Step 2: Evaluate each option for correctness and organization

   router_products = APIRouter(prefix='/products', tags=['products'])\nrouter_orders = APIRouter(prefix='/orders', tags=['orders']) correctly assigns string prefixes and list tags for both routers, organizing API paths and documentation clearly.

3. Final Answer:

   router_products = APIRouter(prefix='/products', tags=['products']) router_orders = APIRouter(prefix='/orders', tags=['orders']) -> Option A

4. Quick Check:

   Correct prefix and tags keep API organized [OK]

Hint: Use string prefix and list tags for each router [OK]

Common Mistakes:

• Using list for prefix or string for tags
• Omitting prefix or tags on one router
• Mixing tags and prefix types