Optional query parameters in FastAPI

Introduction

Optional query parameters let users send extra information in a web request, but they don't have to. This makes your app flexible and user-friendly.

When you want to filter results but don't require the filter every time.

When you want to allow users to sort data optionally.

When you want to add search keywords that users can skip.

When you want to provide pagination controls that users can omit.

When you want to accept extra info that changes output but isn't mandatory.

Syntax

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

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

Use Optional[type] from typing to mark a query parameter as optional.

Set the default value to None to make the parameter optional.

Examples

This example shows a simple optional string query parameter q. If not given, it returns a default message.

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get("/search/")
async def search_items(q: Optional[str] = None):
    return {"search": q or "No search term"}

Here, category is optional and can be skipped. limit has a default value of 10 if not provided.

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get("/products/")
async def get_products(category: Optional[str] = None, limit: int = 10):
    return {"category": category, "limit": limit}

Sample Program

This FastAPI app defines one endpoint /items/ with an optional query parameter q. If the user sends ?q=apple, it returns a message with the search term. If q is missing, it returns a default message.

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get("/items/")
async def read_items(q: Optional[str] = None):
    if q:
        return {"message": f"You searched for: {q}"}
    return {"message": "No search query provided"}

Important Notes

Optional query parameters help keep your API flexible and easy to use.

Always set a default value (usually None) to make a parameter optional.

FastAPI automatically documents optional parameters in the API docs.

Summary

Optional query parameters let users send extra info but don't have to.

Use Optional[type] and default None to make parameters optional.

This makes your API flexible and user-friendly.

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