Basic query parameter declaration in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when accessing the endpoint with ?q=hello?

Consider this FastAPI endpoint:

from fastapi import FastAPI

app = FastAPI()

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

What will the response be when you visit /items/?q=hello?

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

A {"query": null}

B {"query": "hello"}

C {"error": "Missing query parameter"}

D {"query": "q=hello"}

💡 Hint

Query parameters are passed after the ? in the URL and matched by name.

The parameter q is declared as a string query parameter. When you visit /items/?q=hello, FastAPI assigns q the value "hello". The function returns a dictionary with key query and value hello.

📝 Syntax

intermediate

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

In FastAPI, how do you declare a query parameter q that is optional and defaults to None?

A async def read_items(q: Optional[str]):

B async def read_items(q: str):

C async def read_items(q: str = None):

D async def read_items(q: str = ''):

💡 Hint

Optional parameters need a default value in Python function definitions.

In FastAPI, to make a query parameter optional, you assign it a default value like None. Option C does this correctly. Option C is missing the default value and will cause an error unless Optional is imported and default is set. Option C is required. Option C defaults to empty string, which is not None.

❓ state_output

advanced

What is the output when no query parameter is provided?

Given this FastAPI endpoint:

from fastapi import FastAPI

app = FastAPI()

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

What will be the response when you visit /search/ without any query parameters?

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

A {"query": "default"}

B {"error": "Missing query parameter"}

C {"query": null}

D {"query": ""}

💡 Hint

Default values are used when no value is provided.

The parameter q has a default value of "default". When no query parameter is given, FastAPI uses this default. So the response contains {"query": "default"}.

🔧 Debug

advanced

Which option causes a runtime error when calling the endpoint?

Consider these FastAPI endpoint definitions. Which one will cause a runtime error when accessed without query parameters?

A async def get_data(q: Optional[str]): return {"q": q}

B async def get_data(q: str = None): return {"q": q}

C async def get_data(q: str = ""): return {"q": q}

D async def get_data(q: str): return {"q": q}

💡 Hint

Check if the parameter has a default value or is optional.

Option A uses Optional[str] but does not provide a default value, so FastAPI treats q as required. Calling without query parameter causes a 422 error. Option A is required but will cause a validation error, not runtime error. Option A and C have defaults and work fine.

🧠 Conceptual

expert

How does FastAPI handle multiple query parameters with the same name?

Given this endpoint:

from fastapi import FastAPI
from typing import List

app = FastAPI()

@app.get("/items/")
async def read_items(q: List[str] = []):
    return {"queries": q}

What will be the output when the URL is /items/?q=foo&q=bar&q=baz?

FastAPI

from fastapi import FastAPI
from typing import List

app = FastAPI()

@app.get("/items/")
async def read_items(q: List[str] = []):
    return {"queries": q}

A {"queries": []}

B {"queries": "foo,bar,baz"}

C {"queries": ["foo"]}

D {"queries": ["foo", "bar", "baz"]}

💡 Hint

FastAPI can collect multiple query parameters with the same name into a list.

When a query parameter is declared as a list, FastAPI collects all values with that name into the list. So q=foo&q=bar&q=baz becomes ["foo", "bar", "baz"].