Boolean query parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

How does FastAPI parse boolean query parameters?

Consider this FastAPI endpoint:

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(active: bool = Query(False)):
    return {"active": active}

What will be the output if you call /items/?active=true?

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/items/")
async def read_items(active: bool = Query(False)):
    return {"active": active}

A {"active": "true"}

B {"active": true}

C {"active": false}

D HTTP 422 Unprocessable Entity error

💡 Hint

FastAPI automatically converts query parameters to the declared type.

FastAPI converts the string 'true' in the query parameter to the boolean True automatically.

📝 Syntax

intermediate

Correct way to declare optional boolean query parameter with default

Which of the following is the correct way to declare an optional boolean query parameter named enabled with default True in FastAPI?

A async def endpoint(enabled: bool = Query(default=True)):

B async def endpoint(enabled: Optional[bool] = Query(True)):

C async def endpoint(enabled: bool = True):

D async def endpoint(enabled: bool = Query(True, alias='enabled')):

💡 Hint

Use Query to specify metadata and default values for query parameters.

Option A correctly uses Query with default=True to declare an optional boolean query parameter with default True.

🔧 Debug

advanced

Why does this boolean query parameter always return False?

Given this FastAPI endpoint:

from fastapi import FastAPI

app = FastAPI()

@app.get("/check/")
async def check_flag(flag: bool = False):
    return {"flag": flag}

If you call /check/?flag=0 or /check/?flag=false, the response is always {"flag": false}. But calling /check/?flag=1 returns {"flag": true}. Why does flag=0 and flag=false both return False?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/check/")
async def check_flag(flag: bool = False):
    return {"flag": flag}

A FastAPI raises an error for '0' but not for 'false'.

B FastAPI treats any string as True except empty strings.

C FastAPI treats '0' and 'false' as False boolean values by design.

D FastAPI ignores query parameters named 'flag'.

💡 Hint

Check how FastAPI converts common string values to booleans.

FastAPI converts '0', 'false', 'False', 'no', 'off' to False boolean values automatically.

❓ state_output

advanced

What is the output when boolean query parameter is missing?

Given this FastAPI endpoint:

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/status/")
async def status(online: bool = Query(...)):
    return {"online": online}

What happens if you call /status/ without the online query parameter?

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/status/")
async def status(online: bool = Query(...)):
    return {"online": online}

A Returns HTTP 422 Unprocessable Entity error because parameter is required

B Returns {"online": false} because default is False

C Returns {"online": true} because default is True

D Returns {"online": null} because parameter is missing

💡 Hint

Query(...) means the parameter is required.

Using Query(...) makes the parameter required, so missing it causes a 422 error.

🧠 Conceptual

expert

How does FastAPI handle boolean query parameters with multiple values?

Consider this FastAPI endpoint:

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get("/flags/")
async def flags(active: List[bool] = Query([])):
    return {"active": active}

What is the output of calling /flags/?active=true&active=false&active=1?

FastAPI

from fastapi import FastAPI, Query
from typing import List

app = FastAPI()

@app.get("/flags/")
async def flags(active: List[bool] = Query([])):
    return {"active": active}

A {"active": [true]}

B {"active": ["true", "false", "1"]}

C HTTP 422 Unprocessable Entity error

D {"active": [true, false, true]}

💡 Hint

FastAPI converts each query parameter value to the declared type in the list.

FastAPI converts each 'active' query parameter string to boolean values in the list.