Default values in FastAPI - Mini Project: Build & Apply

Using Default Values in FastAPI Query Parameters

📖 Scenario: You are building a simple web API that returns information about books. Sometimes users want to filter books by genre, but if they don't specify a genre, the API should return all books.

🎯 Goal: Create a FastAPI app with a GET endpoint /books that accepts an optional query parameter genre with a default value of "all". The endpoint should return a list of books filtered by the genre if specified, or all books if the genre is "all".

📋 What You'll Learn

Create a list variable called books containing dictionaries with keys title and genre.

Create a variable called default_genre with the value "all".

Create a GET endpoint /books with a query parameter genre that defaults to default_genre.

Filter the books list by genre if it is not "all", otherwise return all books.

💡 Why This Matters

🌍 Real World

APIs often need to handle optional filters with default values to provide flexible data retrieval for users.

💼 Career

Understanding default values in query parameters is essential for backend developers building user-friendly and robust APIs.

1

Create the initial books data

Create a list called books with these exact dictionaries: {"title": "The Hobbit", "genre": "fantasy"}, {"title": "1984", "genre": "dystopian"}, and {"title": "Python 101", "genre": "education"}.

FastAPI

# Create the books list with the given dictionaries
# Your code here

Need a hint?

Use a list with dictionaries. Each dictionary has keys title and genre.

2

Add a default genre variable

Create a variable called default_genre and set it to the string "all".

FastAPI

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
# Create default_genre variable with value "all"
# Your code here

Need a hint?

Just assign the string "all" to default_genre.

3

Create the FastAPI app and GET endpoint with default query parameter

Import FastAPI and Query from fastapi. Create a FastAPI app called app. Define a GET endpoint /books with a query parameter genre that defaults to default_genre using Query(default_genre).

FastAPI

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
default_genre = "all"

# Import FastAPI and Query, create app, and define GET /books endpoint with genre query parameter defaulting to default_genre
# Your code here

Need a hint?

Use Query(default_genre) to set the default value for the query parameter.

4

Filter books by genre or return all

Inside the get_books function, return a list of books filtered by genre if genre is not "all". Otherwise, return the full books list.

FastAPI

from fastapi import FastAPI, Query

books = [
    {"title": "The Hobbit", "genre": "fantasy"},
    {"title": "1984", "genre": "dystopian"},
    {"title": "Python 101", "genre": "education"}
]
default_genre = "all"

app = FastAPI()

@app.get("/books")
async def get_books(genre: str = Query(default_genre)):
    # Return filtered books if genre is not "all", else return all books
    # Your code here

Need a hint?

Use a list comprehension to filter books by genre.