Path parameter types and validation in FastAPI - Mini Project: Build & Apply

Path Parameter Types and Validation with FastAPI

📖 Scenario: You are building a simple web API for a bookstore. The API needs to get book details by their ID and filter books by their rating.

🎯 Goal: Create a FastAPI app that uses path parameters with specific types and validation to ensure correct input from users.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a path parameter book_id as an integer in the route /books/{book_id}

Add validation to book_id so it must be greater than 0

Define a path parameter rating as a float in the route /books/rating/{rating}

Add validation to rating so it must be between 0.0 and 5.0 inclusive

Return simple JSON responses confirming the received parameters

💡 Why This Matters

🌍 Real World

APIs often need to accept user input in URLs. Validating these inputs ensures the API works correctly and securely.

💼 Career

Understanding path parameter types and validation is essential for backend developers building reliable web services with FastAPI.

1

Create FastAPI app and integer path parameter

Import FastAPI from fastapi and create an app instance called app. Then define a GET route /books/{book_id} with a path parameter book_id of type int. The function should return a dictionary with the key book_id and its value.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Define GET route /books/{book_id} with int path parameter book_id
# Your code here

Need a hint?

Use app = FastAPI() to create the app. Define a function with book_id: int as parameter and decorate it with @app.get("/books/{book_id}").

2

Add validation for book_id to be greater than 0

Import Path from fastapi. Update the book_id parameter to use Path(..., gt=0) to enforce that book_id must be greater than 0.

FastAPI

from fastapi import FastAPI, Path

app = FastAPI()

@app.get("/books/{book_id}")
async def read_book(book_id: int = Path(..., gt=0)):
    return {"book_id": book_id}

# Add Path validation to book_id to be greater than 0
# Your code here

Need a hint?

Use Path(..., gt=0) to require book_id to be greater than zero.

3

Add float path parameter with range validation

Define a new GET route /books/rating/{rating} with a path parameter rating of type float. Use Path(..., ge=0.0, le=5.0) to validate that rating is between 0.0 and 5.0 inclusive. The function should return a dictionary with the key rating and its value.

FastAPI

from fastapi import FastAPI, Path

app = FastAPI()

@app.get("/books/{book_id}")
async def read_book(book_id: int = Path(..., gt=0)):
    return {"book_id": book_id}

# Define GET route /books/rating/{rating} with float path parameter rating
# Use Path validation to ensure rating is between 0.0 and 5.0 inclusive
# Your code here

Need a hint?

Define a new async function with rating: float = Path(..., ge=0.0, le=5.0) and decorate it with @app.get("/books/rating/{rating}").

4

Complete FastAPI app with both routes and validation

Ensure the FastAPI app has both routes: /books/{book_id} with book_id as an integer path parameter validated to be greater than 0, and /books/rating/{rating} with rating as a float path parameter validated between 0.0 and 5.0 inclusive. Both routes should return JSON with the received parameter.

FastAPI

from fastapi import FastAPI, Path

app = FastAPI()

@app.get("/books/{book_id}")
async def read_book(book_id: int = Path(..., gt=0)):
    return {"book_id": book_id}

@app.get("/books/rating/{rating}")
async def read_books_by_rating(rating: float = Path(..., ge=0.0, le=5.0)):
    return {"rating": rating}

# Finalize the app with both routes and validations
# Your code here

Need a hint?

Make sure both routes and their validations are present in the app code.