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

Required Query Parameters in FastAPI

📖 Scenario: You are building a simple web API for a book store. You want to create an endpoint that requires users to provide the book_id as a query parameter to get details about a specific book.

🎯 Goal: Create a FastAPI endpoint that requires a book_id query parameter. The endpoint should return a JSON response with the book_id provided.

📋 What You'll Learn

Create a FastAPI app instance named app

Create a GET endpoint at path /book

Require a query parameter named book_id of type int

Return a dictionary with the key book_id and the value from the query parameter

💡 Why This Matters

🌍 Real World

APIs often require clients to send specific information as query parameters to fetch or filter data. This project shows how to enforce required query parameters in FastAPI.

💼 Career

Knowing how to handle required query parameters is essential for backend developers building RESTful APIs with FastAPI, a popular modern Python framework.

1

Create FastAPI app instance

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

FastAPI

from fastapi import FastAPI

# Create FastAPI app instance
# Your code here

Hint

Use FastAPI() to create the app instance and assign it to app.

2

Define GET endpoint with required query parameter

Define a GET endpoint at path /book using @app.get("/book"). Add a function named get_book that takes a required query parameter book_id of type int.

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Define GET endpoint at /book with required query parameter book_id
# Your code here

Hint

Use function parameter typing to require book_id as an integer query parameter.

3

Return book_id in response

Inside the get_book function, return a dictionary with the key "book_id" and the value from the book_id parameter.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/book")
def get_book(book_id: int):
    # Return book_id in a dictionary
    # Your code here

Hint

Return a dictionary with "book_id" as key and the function parameter book_id as value.

4

Add type hint and run the app

Ensure the book_id parameter is typed as int and the app is ready to run. (No extra code needed to run here, just confirm the final code structure.)

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/book")
def get_book(book_id: int):
    return {"book_id": book_id}

# Finalize the code structure
# Your code here

Hint

The code is complete and ready to run with uvicorn or similar server.

Practice

1. In FastAPI, how do you make a query parameter required so the client must provide it in the URL?

easy

A. Use Optional type hint

B. Set the parameter default to None

C. Use Query(...) in the function parameter

D. Use Body(...) instead of query

Solution

1. Step 1: Understand FastAPI query parameters

   FastAPI uses function parameters to define query parameters automatically.

2. Step 2: Use Query(...) to require parameter

   Using Query(...) tells FastAPI this parameter is required and must be in the URL.

3. Final Answer:

   Use Query(...) in the function parameter -> Option C

4. Quick Check:

   Required query parameter = Query(...) [OK]

Hint: Use Query(...) to make query parameters required [OK]

Common Mistakes:

• Setting default to None makes parameter optional
• Using Optional type hint allows missing parameter
• Using Body(...) defines body, not query parameter

2. Which of the following is the correct syntax to declare a required query parameter named item_id of type int in FastAPI?

easy

A. def read_item(item_id: int = Query(None)):

B. def read_item(item_id: int = None):

C. def read_item(item_id: int = Body(...)):

D. def read_item(item_id: int = Query(...)):

Solution

1. Step 1: Identify required query parameter syntax

   To make a query parameter required, use Query(...) as the default value.

2. Step 2: Check each option

   def read_item(item_id: int = Query(...)): uses Query(...) correctly, making item_id required. def read_item(item_id: int = Query(None)): uses Query(None) which makes it optional. def read_item(item_id: int = None): uses default None without Query, optional. def read_item(item_id: int = Body(...)): uses Body(...) which makes it a required body field, not query parameter.

3. Final Answer:

   def read_item(item_id: int = Query(...)): -> Option D

4. Quick Check:

   Required query param = Query(...) [OK]

Hint: Use Query(...) as default to require query parameter [OK]

Common Mistakes:

• Using Query(None) makes parameter optional
• Using Body(...) for query parameter
• Setting default None makes parameter optional

3. Given this FastAPI endpoint code:

from fastapi import FastAPI, Query
app = FastAPI()

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

What happens if a client calls /items/ without the q query parameter?

medium

A. Returns HTTP 422 error for missing required query parameter

B. Returns HTTP 404 not found error

C. Returns empty JSON {}

D. Returns JSON with {"query": null}

Solution

1. Step 1: Understand Query(...) behavior

   Using Query(...) makes the parameter required. If missing, FastAPI returns an error automatically.

2. Step 2: Check what error FastAPI returns

   FastAPI returns HTTP 422 Unprocessable Entity error when a required query parameter is missing.

3. Final Answer:

   Returns HTTP 422 error for missing required query parameter -> Option A

4. Quick Check:

   Missing required query param = HTTP 422 error [OK]

Hint: Missing required query param causes HTTP 422 error [OK]

Common Mistakes:

• Expecting null or empty JSON instead of error
• Confusing 404 not found with parameter error
• Assuming optional parameter behavior

4. Identify the error in this FastAPI code snippet that tries to require a query parameter name:

from fastapi import FastAPI, Query
app = FastAPI()

@app.get("/users/")
async def get_user(name: str = Query(None)):
    return {"user": name}

medium

A. Missing type hint for name

B. The parameter is optional, not required

C. Query(...) should be replaced with Query(None)

D. Function should not be async

Solution

1. Step 1: Analyze Query parameter default

   The code uses Query(None) which means the parameter is optional.

2. Step 2: Check requirement for required parameter

   To make it required, it should use Query(...) instead of Query(None).

3. Final Answer:

   The parameter is optional, not required -> Option B

4. Quick Check:

   Query(None) = optional parameter [OK]

Hint: Query(None) means optional; use Query(...) for required [OK]

Common Mistakes:

• Confusing Query(None) with required parameter
• Ignoring type hints
• Thinking async keyword affects parameter requirement

5. You want to create a FastAPI endpoint that requires two query parameters: category (string) and limit (integer). Both must be required. Which code snippet correctly enforces this?

hard

A. async def get_items(category: str = Query(...), limit: int = Query(...)):

B. async def get_items(category: str = None, limit: int = None):

C. async def get_items(category: str, limit: int = Query(None)):

D. async def get_items(category: str = Query(None), limit: int = Query(...)):

Solution

1. Step 1: Understand required query parameters

   Both parameters must use Query(...) to be required.

2. Step 2: Evaluate each option

   async def get_items(category: str = Query(...), limit: int = Query(...)): correctly uses Query(...) for both. async def get_items(category: str = None, limit: int = None) uses None defaults, making them optional. async def get_items(category: str, limit: int = Query(None)) makes only limit optional. async def get_items(category: str = Query(None), limit: int = Query(...)) makes category optional.

3. Final Answer:

   async def get_items(category: str = Query(...), limit: int = Query(...)): -> Option A

4. Quick Check:

   Both required = Query(...) for each [OK]

Hint: Use Query(...) for each required query parameter [OK]

Common Mistakes:

• Setting default None makes parameter optional
• Mixing required and optional unintentionally
• Omitting Query makes parameter a path parameter