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

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

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

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

Hint

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

Practice

1. What is the main purpose of query parameters in a FastAPI endpoint?

easy

A. To filter or customize the data returned by the API

B. To define the HTTP method used by the endpoint

C. To specify the database connection details

D. To set the server port number

Solution

1. Step 1: Understand what query parameters do

   Query parameters are used to send extra information in the URL to control what data the API returns.

2. Step 2: Identify the purpose in FastAPI

   In FastAPI, query parameters let users filter or customize the response without changing the endpoint itself.

3. Final Answer:

   To filter or customize the data returned by the API -> Option A

4. Quick Check:

   Query parameters = filter/customize data [OK]

Hint: Query parameters control what data you get back [OK]

Common Mistakes:

• Confusing query parameters with HTTP methods
• Thinking query parameters set server or database settings
• Assuming query parameters change endpoint URLs

2. Which of the following is the correct way to declare an optional query parameter named category in FastAPI?

easy

A. def read_items(category: str):

B. def read_items(category: str = None):

C. def read_items(category):

D. def read_items(category: int = 0):

Solution

1. Step 1: Recall how to declare optional query parameters

   In FastAPI, optional query parameters are declared by giving a default value, often None.

2. Step 2: Check the options for correct syntax

   def read_items(category: str = None): uses category: str = None, which means the parameter is optional and of type string.

3. Final Answer:

   def read_items(category: str = None): -> Option B

4. Quick Check:

   Optional query param = default None [OK]

Hint: Optional query params have default values like None [OK]

Common Mistakes:

• Not providing a default value for optional parameters
• Using wrong data types for query parameters
• Omitting type annotations

3. Given this FastAPI endpoint:

from fastapi import FastAPI
app = FastAPI()

items = [
  {"name": "apple", "category": "fruit"},
  {"name": "carrot", "category": "vegetable"}
]

@app.get("/items")
async def read_items(category: str = None):
    if category:
        return [item for item in items if item["category"] == category]
    return items

What will be the output of a GET request to /items?category=fruit?

medium

A. [{"name": "apple", "category": "fruit"}]

B. [{"name": "carrot", "category": "vegetable"}]

C. []

D. [{"name": "apple", "category": "fruit"}, {"name": "carrot", "category": "vegetable"}]

Solution

1. Step 1: Understand the query parameter usage

   The endpoint checks if category is given, then filters items matching that category.

2. Step 2: Apply the filter for category='fruit'

   Only the item with category 'fruit' is the apple, so the returned list contains only that item.

3. Final Answer:

   [{"name": "apple", "category": "fruit"}] -> Option A

4. Quick Check:

   Filter by category 'fruit' returns apple [OK]

Hint: Filter returns only items matching query parameter value [OK]

Common Mistakes:

• Returning all items ignoring the filter
• Returning empty list when filter matches
• Confusing category values

4. Identify the error in this FastAPI endpoint that tries to filter items by a query parameter color:

from fastapi import FastAPI
app = FastAPI()

items = [
  {"name": "shirt", "color": "blue"},
  {"name": "pants", "color": "black"}
]

@app.get("/items")
def read_items(color):
    return [item for item in items if item["color"] == color]

medium

A. The items list is empty

B. The list comprehension syntax is incorrect

C. The endpoint should use POST instead of GET

D. The query parameter 'color' is missing a type annotation and default value

Solution

1. Step 1: Check function parameter declaration

   The parameter 'color' lacks a type annotation and default value, so FastAPI treats it as a required path parameter, causing errors.

2. Step 2: Correct declaration for query parameter

   Adding a type (e.g., str) and a default (e.g., None) makes it an optional query parameter, fixing the error.

3. Final Answer:

   The query parameter 'color' is missing a type annotation and default value -> Option D

4. Quick Check:

   Query params need type and default [OK]

Hint: Query params must have type and default to work [OK]

Common Mistakes:

• Omitting type annotations for query parameters
• Confusing query parameters with path parameters
• Using wrong HTTP methods for filtering

5. You want to create a FastAPI endpoint that returns products filtered by optional query parameters category and max_price. Which code snippet correctly implements this filtering?

hard

A. async def get_products(category: str, max_price: float):
return products

B. async def get_products(category, max_price):
return [p for p in products if p['category'] == category and p['price'] <= max_price]

C. async def get_products(category: str = None, max_price: float = None):
filtered = products
if category:
filtered = [p for p in filtered if p['category'] == category]
if max_price is not None:
filtered = [p for p in filtered if p['price'] <= max_price]
return filtered

D. async def get_products():
return products

Solution

1. Step 1: Understand optional query parameters usage

   Both category and max_price are optional, so they need default values (None) to allow filtering only when provided.

2. Step 2: Check filtering logic

   async def get_products(category: str = None, max_price: float = None): filtered = products if category: filtered = [p for p in filtered if p['category'] == category] if max_price is not None: filtered = [p for p in filtered if p['price'] <= max_price] return filtered filters products step-by-step, applying each filter only if the parameter is given, which is correct and safe.

3. Final Answer:

   async def get_products(category: str = None, max_price: float = None): filtered = products if category: filtered = [p for p in filtered if p['category'] == category] if max_price is not None: filtered = [p for p in filtered if p['price'] <= max_price] return filtered -> Option C

4. Quick Check:

   Optional filters with defaults and conditions = correct [OK]

Hint: Use defaults None and conditionally filter inside function [OK]

Common Mistakes:

• Not providing default values for optional parameters
• Filtering without checking if parameters exist
• Returning unfiltered data ignoring query parameters