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

Optional Query Parameters in FastAPI

📖 Scenario: You are building a simple web API for a bookstore. You want to allow users to search for books by title. Additionally, users can optionally filter the search results by author name if they want.

🎯 Goal: Create a FastAPI app with an endpoint /books/ that accepts a required query parameter title and an optional query parameter author. The endpoint should return a JSON list of books matching the title and, if provided, the author.

📋 What You'll Learn

Create a FastAPI app instance named app

Define a GET endpoint at /books/

Use a required query parameter title of type str

Use an optional query parameter author of type str with default null

Return a filtered list of books matching the title and optionally the author

💡 Why This Matters

🌍 Real World

APIs often need to accept optional filters to let users narrow down search results without requiring all filters.

💼 Career

Understanding optional query parameters is essential for backend developers building flexible and user-friendly APIs.

1

Create the initial data list of books

Create a list called books with these dictionaries exactly: {'title': 'The Hobbit', 'author': 'J.R.R. Tolkien'}, {'title': '1984', 'author': 'George Orwell'}, {'title': 'The Silmarillion', 'author': 'J.R.R. Tolkien'}

FastAPI

# Create the list of books with exact entries
# Your code here

Hint

Use a list of dictionaries. Each dictionary has keys 'title' and 'author'.

2

Create the FastAPI app and import required modules

Import FastAPI from fastapi and create an app instance called app

FastAPI

books = [
    {'title': 'The Hobbit', 'author': 'J.R.R. Tolkien'},
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'The Silmarillion', 'author': 'J.R.R. Tolkien'}
]

# Import FastAPI and create app instance
# Your code here

Hint

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

3

Define the GET endpoint with required and optional query parameters

Define a GET endpoint /books/ using @app.get("/books/"). Create a function read_books with parameters title: str and author: str | null = null to accept required title and optional author query parameters.

FastAPI

from fastapi import FastAPI

books = [
    {'title': 'The Hobbit', 'author': 'J.R.R. Tolkien'},
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'The Silmarillion', 'author': 'J.R.R. Tolkien'}
]

app = FastAPI()

# Define GET endpoint /books/ with parameters title and optional author
# Your code here

Hint

Use @app.get("/books/") and define read_books with parameters title: str and author: str | null = null.

4

Filter and return books matching title and optional author

Inside read_books, create a list results that includes books from books where book['title'] contains title (case insensitive). If author is not null, further filter results to include only books where book['author'] contains author (case insensitive). Return results.

FastAPI

from fastapi import FastAPI

books = [
    {'title': 'The Hobbit', 'author': 'J.R.R. Tolkien'},
    {'title': '1984', 'author': 'George Orwell'},
    {'title': 'The Silmarillion', 'author': 'J.R.R. Tolkien'}
]

app = FastAPI()

@app.get("/books/")
def read_books(title: str, author: str | null = null):
    # Filter books by title and optional author
    # Your code here

Hint

Use list comprehensions to filter books by title and optionally by author. Use str.lower() for case-insensitive matching.

Practice

1. What does it mean when a query parameter in FastAPI is optional?

easy

A. The parameter is required but can be empty

B. The parameter must always be included in the request

C. The user can choose to include it or not in the request

D. The parameter is ignored by FastAPI

Solution

1. Step 1: Understand optional parameters in APIs

   Optional means the user does not have to provide the parameter for the request to work.

2. Step 2: Apply to FastAPI query parameters

   FastAPI allows parameters to be optional by giving them a default value like None.

3. Final Answer:

   The user can choose to include it or not in the request -> Option C

4. Quick Check:

   Optional query parameter = user choice [OK]

Hint: Optional means user may skip it without error [OK]

Common Mistakes:

• Thinking optional means parameter is always required
• Confusing optional with ignored parameters
• Assuming optional means empty string is required

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

easy

A. def read_items(q: Optional[str] = None):

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

C. def read_items(q: str):

D. def read_items(q: Optional[str]):

Solution

1. Step 1: Recall syntax for optional query parameters

   FastAPI uses Optional[type] with a default value None to mark parameters optional.

2. Step 2: Check each option

   def read_items(q: Optional[str] = None): uses Optional[str] and default None, which is correct syntax.

3. Final Answer:

   def read_items(q: Optional[str] = None): -> Option A

4. Quick Check:

   Optional[str] + = None = correct syntax [OK]

Hint: Use Optional[type] with = None for optional params [OK]

Common Mistakes:

• Omitting the default None value
• Using Optional without import
• Setting type to str without Optional

3. Given this FastAPI endpoint code:

from typing import Optional
from fastapi import FastAPI

app = FastAPI()

@app.get("/items/")
async def read_items(q: Optional[str] = None):
    if q:
        return {"query": q}
    return {"query": "No query"}

What will be the JSON response when the client calls /items/ without any query parameters?

medium

A. {"query": null}

B. {"query": "No query"}

C. An error occurs because q is missing

D. {"query": ""}

Solution

1. Step 1: Understand default value of q

   q is Optional[str] with default None, so if missing, q is None.

2. Step 2: Check the if condition

   If q is None (falsey), the function returns {"query": "No query"}.

3. Final Answer:

   {"query": "No query"} -> Option B

4. Quick Check:

   Missing q returns "No query" [OK]

Hint: Missing optional param uses default None [OK]

Common Mistakes:

• Expecting error when param is missing
• Assuming null is returned instead of string
• Confusing empty string with None

4. Identify the error in this FastAPI endpoint code that tries to make q optional:

from fastapi import FastAPI

app = FastAPI()

@app.get("/search/")
def search(q: str = None):
    return {"q": q}

medium

A. q should be typed as Optional[str] when default is None

B. Function must be async to use optional parameters

C. Missing import of Optional from typing

D. Default value None is not allowed for query parameters

Solution

1. Step 1: Check parameter typing and default

   q has type str but default None, which conflicts because None is not str.

2. Step 2: Correct typing for optional parameter

   Use Optional[str] from typing to allow None as a valid default.

3. Final Answer:

   q should be typed as Optional[str] when default is None -> Option A

4. Quick Check:

   Type must match default None with Optional[str] [OK]

Hint: Match type with default None using Optional[type] [OK]

Common Mistakes:

• Not importing Optional but using it
• Making function async unnecessarily
• Using None default without Optional type

5. You want to create a FastAPI endpoint that accepts an optional integer query parameter page with a default of 1, but if the user sends page=0, it should return an error. How would you declare page to achieve this behavior?

hard

A. page: Optional[int] = 1 with extra validation to reject 0

B. page: int without default and require user to send page > 0

C. page: Optional[int] = None and set page = 1 inside function if None

D. page: int = 1 and check inside function if page == 0 to raise error

Solution

1. Step 1: Understand default and validation needs

   Default 1 means page is not optional, but user can omit it to get 1.

2. Step 2: Handle invalid value 0

   Use int with default 1, then inside function check if page == 0 and raise error.

3. Final Answer:

   page: int = 1 and check inside function if page == 0 to raise error -> Option D

4. Quick Check:

   Default + validation inside function = page: int = 1 and check inside function if page == 0 to raise error [OK]

Hint: Use default param + check value inside function [OK]

Common Mistakes:

• Making page Optional[int] = 1 (invalid syntax)
• Not validating 0 value inside function
• Requiring page without default when default needed