Why query parameters filter data in FastAPI - See It in Action

Why Query Parameters Filter Data

📖 Scenario: You are building a simple web API for a bookstore. Customers want to see books filtered by genre using query parameters in the URL.

🎯 Goal: Create a FastAPI app that returns a list of books. Add a query parameter to filter books by genre.

📋 What You'll Learn

Create a list of books with title and genre

Add a query parameter called genre to filter books

Return all books if no genre is specified

Use FastAPI's Query for the genre parameter

💡 Why This Matters

🌍 Real World

Filtering data with query parameters is common in web APIs to let users get only the data they want.

💼 Career

Understanding query parameters and filtering is essential for backend developers building APIs.

1

Create the initial list of books

Create a list called books with these dictionaries exactly: {'title': 'The Hobbit', 'genre': 'Fantasy'}, {'title': '1984', 'genre': 'Dystopian'}, {'title': 'The Catcher in the Rye', 'genre': 'Classic'}.

FastAPI

# Create the list of books
# Your code here

Need a hint?

Use a list with dictionaries for each book.

2

Add FastAPI app and import Query

Import FastAPI and Query from fastapi. Create an app instance called app.

FastAPI

books = [
    {'title': 'The Hobbit', 'genre': 'Fantasy'},
    {'title': '1984', 'genre': 'Dystopian'},
    {'title': 'The Catcher in the Rye', 'genre': 'Classic'}
]

# Import FastAPI and Query, create app
# Your code here

Need a hint?

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

3

Create the route with genre query parameter

Create a GET route /books with a function get_books that has a query parameter genre of type str | None with default None using Query(None). Inside the function, filter books to only those with matching genre if genre is not None, else return all books.

FastAPI

from fastapi import FastAPI, Query

books = [
    {'title': 'The Hobbit', 'genre': 'Fantasy'},
    {'title': '1984', 'genre': 'Dystopian'},
    {'title': 'The Catcher in the Rye', 'genre': 'Classic'}
]

app = FastAPI()

# Create GET /books route with genre query parameter
# Your code here

Need a hint?

Use a list comprehension to filter books by genre.

4

Add response model and complete the app

Import List and Dict from typing. Add a response model to @app.get as List[Dict[str, str]] to show the output is a list of dictionaries with string keys and values.

FastAPI

from fastapi import FastAPI, Query
from typing import List, Dict

books = [
    {'title': 'The Hobbit', 'genre': 'Fantasy'},
    {'title': '1984', 'genre': 'Dystopian'},
    {'title': 'The Catcher in the Rye', 'genre': 'Classic'}
]

app = FastAPI()

# Add response_model to the route
# Your code here

Need a hint?

Use response_model=List[Dict[str, str]] in the route decorator.