Multiple query parameters in FastAPI - Mini Project: Build & Apply

Multiple query parameters

📖 Scenario: You are building a simple web API using FastAPI. Your API will accept multiple query parameters from the URL to filter a list of items.Imagine you have a store with products, and you want users to search products by category and price range using query parameters.

🎯 Goal: Create a FastAPI app that accepts two query parameters: category and max_price. The app should return a filtered list of products matching the category and with price less than or equal to max_price.

📋 What You'll Learn

Create a list of products as dictionaries with keys 'name', 'category', and 'price'.

Add query parameters category (string) and max_price (float) to the GET endpoint.

Filter the products list based on the query parameters.

Return the filtered list as JSON response.

💡 Why This Matters

🌍 Real World

APIs often need to accept multiple query parameters to filter or search data, like searching products by category and price.

💼 Career

Understanding how to handle multiple query parameters in FastAPI is essential for backend web development and building flexible APIs.

1

DATA SETUP: Create the products list

Create a list called products with these exact dictionaries: {'name': 'Apple', 'category': 'fruit', 'price': 1.2}, {'name': 'Banana', 'category': 'fruit', 'price': 0.5}, {'name': 'Carrot', 'category': 'vegetable', 'price': 0.8}, {'name': 'Steak', 'category': 'meat', 'price': 5.0}.

FastAPI

# Create the products list with exact items
# Your code here

Hint

Use a list of dictionaries exactly as shown.

2

CONFIGURATION: Import FastAPI and create app

Import FastAPI from fastapi and create an app instance called app.

FastAPI

products = [
    {'name': 'Apple', 'category': 'fruit', 'price': 1.2},
    {'name': 'Banana', 'category': 'fruit', 'price': 0.5},
    {'name': 'Carrot', 'category': 'vegetable', 'price': 0.8},
    {'name': 'Steak', 'category': 'meat', 'price': 5.0}
]

# Import FastAPI and create app instance
# Your code here

Hint

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

3

CORE LOGIC: Create GET endpoint with query parameters

Create a GET endpoint at /items/ using @app.get("/items/"). Define a function read_items with two query parameters: category: str and max_price: float. Inside the function, filter products to include only items where item['category'] == category and item['price'] <= max_price. Return the filtered list.

FastAPI

from fastapi import FastAPI

app = FastAPI()

products = [
    {'name': 'Apple', 'category': 'fruit', 'price': 1.2},
    {'name': 'Banana', 'category': 'fruit', 'price': 0.5},
    {'name': 'Carrot', 'category': 'vegetable', 'price': 0.8},
    {'name': 'Steak', 'category': 'meat', 'price': 5.0}
]

# Create GET endpoint with query parameters category and max_price
# Your code here

Hint

Use a list comprehension to filter products by category and max_price.

4

COMPLETION: Add type hints and async keyword

Ensure the endpoint function read_items is declared with async def and has type hints for category: str and max_price: float. Confirm the function returns the filtered list of products.

FastAPI

from fastapi import FastAPI

app = FastAPI()

products = [
    {'name': 'Apple', 'category': 'fruit', 'price': 1.2},
    {'name': 'Banana', 'category': 'fruit', 'price': 0.5},
    {'name': 'Carrot', 'category': 'vegetable', 'price': 0.8},
    {'name': 'Steak', 'category': 'meat', 'price': 5.0}
]

@app.get("/items/")
# Confirm async def and type hints
# Your code here

Hint

Make sure the function is async and has correct type hints.

Practice

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

easy

A. To accept several inputs from the URL to customize the response

B. To define multiple routes in one function

C. To handle only POST request data

D. To create database models automatically

Solution

1. Step 1: Understand query parameters in FastAPI

   Query parameters allow passing data in the URL after a question mark, like ?name=John.

2. Step 2: Purpose of multiple query parameters

   Using multiple query parameters lets the API accept many inputs to customize what it returns.

3. Final Answer:

   To accept several inputs from the URL to customize the response -> Option A

4. Quick Check:

   Multiple query parameters = multiple inputs [OK]

Hint: Multiple query parameters let you send many inputs in URL [OK]

Common Mistakes:

• Confusing query parameters with request body
• Thinking multiple query parameters define routes
• Assuming query parameters only work with POST

2. Which of the following is the correct way to declare two query parameters name (string) and age (integer) in a FastAPI path operation function?

easy

A. def read_user(name: str = Query(...), age: int = Query(...)):

B. def read_user(name: str, age: int = None):

C. def read_user(name: int, age: str):

D. def read_user(name, age):

Solution

1. Step 1: Understand FastAPI query parameter declaration

   Using Query(...) marks parameters as required query parameters with type hints.

2. Step 2: Check parameter types and usage

   Parameters must have correct types and use Query(...) to be recognized as query parameters.

3. Final Answer:

   def read_user(name: str = Query(...), age: int = Query(...)): -> Option A

4. Quick Check:

   Use Query(...) with correct types for query parameters [OK]

Hint: Use Query(...) with type hints for required query parameters [OK]

Common Mistakes:

• Swapping types of parameters
• Not using Query for required parameters
• Omitting type hints

3. Given this FastAPI endpoint code:

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get('/items/')
async def read_items(q: List[str] = Query([])):
    return {"q": q}

What will be the output if the URL is /items/?q=apple&q=banana?

medium

A. {"q": ["apple"]}

B. {"q": "apple,banana"}

C. {"q": ["apple", "banana"]}

D. {"q": []}

Solution

1. Step 1: Understand List query parameters in FastAPI

   Using List[str] with Query([]) allows multiple values for the same query key to be collected into a list.

2. Step 2: Analyze the URL query parameters

   The URL has q=apple and q=banana, so FastAPI collects both into a list.

3. Final Answer:

   {"q": ["apple", "banana"]} -> Option C

4. Quick Check:

   Multiple query keys = list of values [OK]

Hint: Multiple same keys in URL become list in FastAPI [OK]

Common Mistakes:

• Expecting a single string with commas
• Getting only the first value
• Getting an empty list

4. What is wrong with this FastAPI endpoint code that tries to accept two query parameters name and age?:

from fastapi import FastAPI
app = FastAPI()

@app.get('/user/')
def get_user(name: str, age: int = None):
    return {"name": name, "age": age}

medium

A. The route path should not end with a slash

B. The function must be async

C. Missing Query() for both parameters

D. The parameter age should use Optional[int] for optional query

Solution

1. Step 1: Check optional query parameter typing

   In FastAPI, optional parameters should use Optional[type] or type | None to mark them optional properly.

2. Step 2: Analyze age: int = None

   Just setting default to None is not enough; the type hint should be Optional[int] or int | None for FastAPI to treat it as optional.

3. Final Answer:

   The parameter age should use Optional[int] for optional query -> Option D

4. Quick Check:

   Optional query params need Optional[type] or type | None [OK]

Hint: Use Optional[int] or int | None for optional query params [OK]

Common Mistakes:

• Assuming default None alone makes param optional
• Forgetting to import Optional
• Thinking async is required for all endpoints

5. You want to create a FastAPI endpoint that accepts multiple optional query parameters: tags (list of strings), limit (integer, default 10), and search (optional string). Which of the following function signatures correctly implements this?

hard

A. def get_items(tags: List[str] = Query([]), limit: int = 10, search: str = None):

B. def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)):

C. def get_items(tags: List[str] = [], limit: int = 10, search: str = ''):

D. def get_items(tags: List[str], limit: int = Query(10), search: Optional[str] = Query(None)):

Solution

1. Step 1: Use Query() for query parameters with defaults

   FastAPI requires using Query() to specify default values and metadata for query parameters.

2. Step 2: Correctly type optional parameters

   Use Optional[str] or str | None with Query(None) for optional string parameters.

3. Step 3: Check all parameters in def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)):

   def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)): uses Query([]) for list default, Query(10) for limit default, and Optional[str] = Query(None) for optional search, which is correct.

4. Final Answer:

   def get_items(tags: List[str] = Query([]), limit: int = Query(10), search: Optional[str] = Query(None)): -> Option B

5. Quick Check:

   Use Query() with correct types and defaults for multiple optional queries [OK]

Hint: Use Query() with defaults and Optional for optional query params [OK]

Common Mistakes:

• Using mutable default [] without Query()
• Not marking optional string with Optional
• Omitting Query() for default values