Optional query parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when no query parameter is provided?

Consider this FastAPI endpoint that uses an optional query parameter with a default value. What will be the response if the client calls the endpoint without any query parameters?

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'}

A {"query": "No query provided"}

B {"query": null}

C 404 Not Found error

D 500 Internal Server Error

💡 Hint

Think about what happens when the optional parameter is not sent by the client.

The parameter 'q' is optional and defaults to None. When no query parameter is sent, 'q' is None, so the function returns {'query': 'No query provided'}.

📝 Syntax

intermediate

Which option correctly declares an optional query parameter with a default value?

You want to declare an optional query parameter named 'page' that defaults to 1. Which of the following FastAPI endpoint parameter declarations is correct?

A async def read_items(page: Optional[int] = 1):

B async def read_items(page: int = 1):

C async def read_items(page: int):

D async def read_items(page: Optional[int]):

💡 Hint

Remember how to set default values for parameters in Python functions.

Declaring 'page: int = 1' makes 'page' optional with a default value of 1. Using Optional[int] without a default value makes it required.

❓ state_output

advanced

What is the response when multiple optional query parameters are missing?

Given this FastAPI endpoint with two optional query parameters, what will be the JSON response if the client calls the endpoint without any query parameters?

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get('/search/')
async def search(q: Optional[str] = None, limit: Optional[int] = 10):
    return {'q': q, 'limit': limit}

A {"q": null, "limit": 10}

B {"q": "", "limit": 0}

C {"q": null, "limit": null}

D 422 Unprocessable Entity error

💡 Hint

Check the default values assigned to the parameters.

Parameter 'q' defaults to None (null in JSON), and 'limit' defaults to 10. So the response includes these values.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error?

Examine the following FastAPI endpoint. Why does it raise a 422 validation error when called without query parameters?

FastAPI

from fastapi import FastAPI
from typing import Optional

app = FastAPI()

@app.get('/items/')
async def read_items(q: Optional[str]):
    return {'query': q}

A Because the return statement is missing a default value.

B Because Optional[str] is not allowed in FastAPI parameters.

C Because the endpoint must have at least one required parameter.

D Because 'q' is Optional but has no default value, so it is required.

💡 Hint

Think about how Python treats parameters without default values.

In Python, parameters without default values are required. 'q: Optional[str]' without '= None' means 'q' is required, so calling without it causes validation error.

🧠 Conceptual

expert

How does FastAPI handle optional query parameters with default values in OpenAPI docs?

When you declare an optional query parameter with a default value in FastAPI, how does it appear in the generated OpenAPI documentation?

A The parameter is marked as required with no default value shown.

B The parameter is hidden from the OpenAPI docs.

C The parameter is marked as optional with the default value shown in the docs.

D The parameter is shown as required but with the default value.

💡 Hint

Think about how default values affect API documentation.

FastAPI uses the default value to mark the parameter as optional in OpenAPI docs and shows the default value to API users.