Multiple query parameters in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when multiple query parameters are passed?

Consider this FastAPI endpoint:

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(q: str, limit: int = 10):
    return {"q": q, "limit": limit}

What will be the JSON response if the URL is /items/?q=book&limit=5?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/items/")
async def read_items(q: str, limit: int = 10):
    return {"q": q, "limit": limit}

A {"q": null, "limit": 5}

B {"q": "book", "limit": 5}

C {"q": "book", "limit": 10}

D HTTP 422 Unprocessable Entity error

💡 Hint

Query parameters in FastAPI are matched by name and type. Default values are used only if the parameter is missing.

The endpoint expects a required string q and an optional integer limit with default 10. Passing q=book and limit=5 sets both accordingly.

📝 Syntax

intermediate

Which option correctly defines multiple query parameters with defaults?

Which FastAPI function signature correctly defines two optional query parameters category (string) and page (integer) with default values?

A async def get_items(category: str = None, page: int = 1):

B async def get_items(category: str = "", page: int):

C async def get_items(category: str, page: int = 1):

D async def get_items(category: str = None, page: int = None):

💡 Hint

Optional query parameters must have default values. Non-optional parameters without defaults are required.

Option A correctly sets category as optional with default None and page as optional with default 1. Option A misses default for page. Option A makes category required. Option A sets page default to None but type is int, which causes validation error.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error?

Given this endpoint:

from fastapi import FastAPI
app = FastAPI()

@app.get("/search")
async def search_items(q: str, limit: int = "10"):
    return {"q": q, "limit": limit}

Why does calling /search?q=phone cause a validation error?

FastAPI

from fastapi import FastAPI
app = FastAPI()

@app.get("/search")
async def search_items(q: str, limit: int = "10"):
    return {"q": q, "limit": limit}

A Because limit is not declared as Optional[int]

B Because q is missing a default value

C Because the endpoint path is missing a trailing slash

D Because limit has a default value as string "10" but should be int

💡 Hint

Check the type of default values compared to the declared parameter type.

The parameter limit is declared as int but its default value is a string "10". FastAPI tries to convert the default but fails validation, causing an error.

❓ state_output

advanced

What is the value of 'tags' after this request?

Consider this FastAPI endpoint:

from fastapi import FastAPI
from typing import List
app = FastAPI()

@app.get("/filter")
async def filter_items(tags: List[str] = []):
    return {"tags": tags}

What will be the output if the URL is /filter?tags=red&tags=blue?

FastAPI

from fastapi import FastAPI
from typing import List
app = FastAPI()

@app.get("/filter")
async def filter_items(tags: List[str] = []):
    return {"tags": tags}

A {"tags": ["red", "blue"]}

B {"tags": ["redblue"]}

C {"tags": []}

D HTTP 422 Unprocessable Entity error

💡 Hint

Multiple query parameters with the same name are parsed as a list.

FastAPI parses repeated query parameters with the same name into a list. So tags=red&tags=blue becomes ["red", "blue"].

🧠 Conceptual

expert

Which option correctly explains how FastAPI handles multiple query parameters with the same name?

When a FastAPI endpoint declares a parameter as tags: List[str], and the request URL contains multiple tags query parameters, how does FastAPI process them?

A FastAPI only takes the first 'tags' query parameter and ignores the rest.

B FastAPI concatenates all 'tags' query parameters into a single string separated by commas.

C FastAPI collects all query parameters named 'tags' into a Python list and passes it to the endpoint parameter.

D FastAPI raises a validation error if multiple 'tags' parameters are present.

💡 Hint

Think about how lists in function parameters relate to repeated query parameters.

FastAPI automatically collects repeated query parameters with the same name into a list when the parameter type is a list. This allows easy handling of multiple values.