Required query parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a required query parameter is missing?

Consider this FastAPI endpoint:

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_item(q: str):
    return {"q": q}

What is the response if the client calls /items/ without the q parameter?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_item(q: str):
    return {"q": q}

A The server returns a 500 Internal Server Error.

B The server returns an empty string for 'q' in the JSON response.

C The server returns a 404 Not Found error.

D The server returns a 422 Unprocessable Entity error because the required query parameter 'q' is missing.

💡 Hint

Think about what FastAPI does when a required parameter is not provided.

FastAPI automatically validates required query parameters. If a required parameter like 'q' is missing, it returns a 422 error indicating the request is invalid.

📝 Syntax

intermediate

Which code correctly defines a required query parameter with a default value?

Which of the following FastAPI endpoint definitions correctly makes q a required query parameter?

A async def read_item(q: str):

B async def read_item(q: str = None):

C async def read_item(q: str = ""):

D async def read_item(q: Optional[str] = None):

💡 Hint

Required parameters do not have default values.

In FastAPI, a query parameter without a default value is required. Adding a default value makes it optional.

❓ state_output

advanced

What is the output when a required query parameter is provided?

Given this FastAPI endpoint:

from fastapi import FastAPI
app = FastAPI()

@app.get("/search")
async def search(q: str):
    return {"query": q.upper()}

What is the JSON response when the client calls /search?q=fastapi?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/search")
async def search(q: str):
    return {"query": q.upper()}

A {"query": "FASTAPI"}

B {"query": "fastapi"}

C 422 Unprocessable Entity error

D {"query": null}

💡 Hint

Look at how the parameter q is used inside the function.

The function returns the uppercase version of the query parameter q. So the response JSON has the value "FASTAPI".

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error?

Examine this code:

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
async def get_items(q: str = None):
    return {"q": q.upper()}

When calling /items without query parameters, what error occurs and why?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items")
async def get_items(q: str = None):
    return {"q": q.upper()}

A 422 Unprocessable Entity error because q is required but missing

B AttributeError because q is None and None has no method upper()

C No error, returns {"q": ""}

D TypeError because q is not a string

💡 Hint

Consider what happens when q is None and you call upper().

Since q defaults to None, calling q.upper() raises an AttributeError because NoneType has no upper method.

🧠 Conceptual

expert

How to enforce a required query parameter with a default value in FastAPI?

In FastAPI, you want a query parameter q that is required but also has a default value if not provided. Which statement is true?

A Use <code>q: str</code> with a validator to assign a default value.

B Use <code>q: str = "default"</code> to make it required with a default.

C You cannot have a required query parameter with a default value; providing a default makes it optional.

D Use <code>q: str = Query(..., default="default")</code> to make it required with a default.

💡 Hint

Think about what 'required' means in FastAPI query parameters.

In FastAPI, if a query parameter has a default value, it is optional. Required means no default value is set.