Route ordering and priority in FastAPI - Mini Project: Build & Apply

Route ordering and priority in FastAPI

📖 Scenario: You are building a simple web API for a bookstore. You want to create routes that respond to different URL paths. Some routes have fixed paths, and others have dynamic parts like book IDs.FastAPI matches routes in the order they are declared. This means the order you write your routes affects which one handles a request.

🎯 Goal: Build a FastAPI app with multiple routes to see how route order affects which route is called. You will create a fixed route and a dynamic route, then reorder them to observe priority.

📋 What You'll Learn

Create a FastAPI app instance called app

Add a fixed route /books/new that returns the text 'Create a new book'

Add a dynamic route /books/{book_id} that returns the text 'Get book with ID {book_id}'

Reorder the routes to see how FastAPI chooses which route to run

💡 Why This Matters

🌍 Real World

Web APIs often have fixed and dynamic routes. Understanding route order helps avoid unexpected behavior when users request URLs.

💼 Career

Backend developers use FastAPI or similar frameworks to build APIs. Knowing route priority is essential to design clear and correct endpoints.

1

Create the FastAPI app and fixed route

Import FastAPI from fastapi. Create an app instance called app. Add a route for the path /books/new using @app.get that returns the string 'Create a new book'.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Add a route for /books/new that returns 'Create a new book'
# Your code here

Need a hint?

Use @app.get('/books/new') to define the fixed route.

2

Add a dynamic route for book IDs

Below the existing code, add a new route for the path /books/{book_id} using @app.get. Define an async function called read_book that takes book_id as a parameter and returns the string 'Get book with ID {book_id}'. Use an f-string to include book_id in the returned string.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get('/books/new')
async def create_book():
    return 'Create a new book'

# Add a dynamic route /books/{book_id} that returns 'Get book with ID {book_id}'
# Your code here

Need a hint?

Use @app.get('/books/{book_id}') and an async function with book_id parameter.

3

Test route order by swapping routes

Swap the order of the two route definitions so that the dynamic route @app.get('/books/{book_id}') comes before the fixed route @app.get('/books/new'). Keep the function definitions the same.

FastAPI

# Swap the order of the routes so dynamic route is first
from fastapi import FastAPI

app = FastAPI()

# Your code here

Need a hint?

Move the @app.get('/books/{book_id}') route above the @app.get('/books/new') route.

4

Add a root route to complete the app

Add a route for the root path / using @app.get. Define an async function called root that returns the string 'Welcome to the Bookstore API'.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get('/books/{book_id}')
async def read_book(book_id: str):
    return f'Get book with ID {book_id}'

@app.get('/books/new')
async def create_book():
    return 'Create a new book'

# Add a root route / that returns 'Welcome to the Bookstore API'
# Your code here

Need a hint?

Use @app.get('/') to add the root route.