Project: Query Parameters

📚 Filtering Books by Category (Query Parameter)

Let’s filter books by category using a query parameter.

from fastapi import FastAPI

app = FastAPI()

BOOKS = ...

#API endpoint that filters BOOKS by category
@app.get("/books/")
async def read_category_by_query(category: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return

🧠 What’s Happening Here?

• 🔁 Loops through all books

• 🔍 Checks for books matching the category from the query parameter

• 📤 Returns a list of matching books

FastAPI automatically detects:

• Anything passed after /books/ that is not part of the path

• Is treated as a query parameter

async def read_category_by_query(category: str):

• 🏷️ category → Query parameter name

• 🔤 str → Only string values are allowed

🌐 Viewing the Result in the Browser

Example: Retrieve books in the math category.

127.0.0.1:8000/books/?category=math 

• ✋ /books/ → Path

• ❓ ?category=math → Query parameter

Returns all books that belong to the math category.

notion (2).mp4

🔗 Combining Path and Query Parameters

Query parameters can be combined with path parameters for more specific queries.

So far:

• 📍 Path parameter → Retrieve books by title or author

• 🔎 Query parameter → Filter books by category

Now, let’s use both together.

🧪 Filtering by Author (Path) and Category (Query)

#API Endpoint with a python function that filters BOOKS by Author and Category
@app.get("/books/{book_author}/")
async def read_author_and_category(book_author: str, category: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('author').casefold() == book_author.casefold() and \\
           book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return

🌐 Viewing the Result in the Browser

Example: Retrieve specific books by a specific author and category

127.0.0.1:8000/books/author%20four/?category=science 

🧠 URL Breakdown

• /books/author%20four → Path parameter (book_author)

• ?category=science → Query parameter (category)

• %20 → Represents a space in the URL

🔄 What This Allows Us to Do

• Retrieve books by a specific author

• Filter results further by category

• Easily try different authors and categories for dynamic responses

• Returns an empty list if no books match

✋ Combining Path + Query parameters is very common in GET requests and makes APIs flexible and user-friendly.

from fastapi import FastAPI

app = FastAPI()

BOOKS = [
    {"title": "Title One", "author": "Author One", "category": "science"},
    {"title": "Title Two", "author": "Author Two", "category": "science"},
    {"title": "Title Three", "author": "Author Three", "category": "history"},
    {"title": "Title Four", "author": "Author Four", "category": "math"},
    {"title": "Title Five", "author": "Author Five", "category": "math"},
]

@app.get("/books")
async def read_all_books():
    return BOOKS

@app.get('/books/{book_title}')
async def read_book(book_title: str):
    for book in BOOKS:
        if book.get('title').casefold() == book_title.casefold():
            return book

@app.get("/books/")
async def read_category_by_query(category: str):
    books_to_return = []
    for book in BOOKS:
        if book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return

@app.get("/books/{book_author}/")
async def read_author_and_category(book_author: str, category: str):
    books_to_return = []
    for book in BOOKS:
        print('book', book)
        if book.get('author').casefold() == book_author.casefold() and \\
                book.get('category').casefold() == category.casefold():
            books_to_return.append(book)
    return books_to_return