Default values in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when calling the endpoint without query parameters?

Consider this FastAPI endpoint using default values for query parameters. What will the response be if you call /items/42 without any query parameters?

FastAPI

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str = "No description provided"

@app.get("/items/{item_id}")
async def read_item(item_id: int, q: str = "default_query"):
    return {"item_id": item_id, "q": q}

A {"item_id": 42, "q": null}

B {"item_id": 42, "q": "default_query"}

C {"item_id": 42}

D Error: Missing required query parameter 'q'

💡 Hint

Think about what happens when a query parameter has a default value and is not provided by the client.

The query parameter 'q' has a default value of 'default_query'. If the client does not provide 'q', FastAPI uses the default. So the response includes 'q' with the default string.

📝 Syntax

intermediate

Which option correctly sets a default value for a path parameter in FastAPI?

FastAPI path parameters cannot have default values. Which of the following code snippets correctly defines a path parameter and a query parameter with a default value?

FastAPI

from fastapi import FastAPI

app = FastAPI()

# Choose the correct snippet:

A

@app.get("/users/{user_id=1}")
async def get_user(user_id: int):
    return {"user_id": user_id}

B

@app.get("/users")
async def get_user(user_id: int = 1):
    return {"user_id": user_id}

C

@app.get("/users/{user_id}")
async def get_user(user_id: int, active: bool = True):
    return {"user_id": user_id, "active": active}

D

@app.get("/users/{user_id}")
async def get_user(user_id: int = 1):
    return {"user_id": user_id}

💡 Hint

Path parameters must always be provided in the URL path. Query parameters can have defaults.

Path parameters cannot have default values because they are part of the URL path and must be specified. Query parameters can have defaults. Option C correctly defines a required path parameter and a query parameter with a default.

❓ state_output

advanced

What is the response when a request omits an optional query parameter with a default value?

Given this FastAPI endpoint, what will be the JSON response if the client calls /search without any query parameters?

FastAPI

from fastapi import FastAPI

app = FastAPI()

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

A {"term": "all", "limit": 10}

B {"term": null, "limit": null}

C {"term": "", "limit": 0}

D 422 Unprocessable Entity error

💡 Hint

Check what happens when query parameters have default values and are not provided.

Both 'term' and 'limit' have default values. If the client omits them, FastAPI uses the defaults. So the response includes 'term' as 'all' and 'limit' as 10.

🔧 Debug

advanced

Why does this FastAPI endpoint raise a validation error?

Examine this FastAPI endpoint. Why does calling /products without query parameters cause a 422 error?

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/products")
async def list_products(category: str):
    return {"category": category}

A Because 'category' is a required query parameter without a default value

B Because path parameters cannot have default values

C Because the endpoint URL is missing a path parameter

D Because the function is missing an async keyword

💡 Hint

Think about which parameters FastAPI expects from the client and which have defaults.

The 'category' parameter is required but has no default. If the client omits it, FastAPI returns a 422 error because it cannot validate the missing required parameter.

🧠 Conceptual

expert

How does FastAPI handle default values for mutable types in query parameters?

Consider this FastAPI endpoint with a query parameter defaulting to an empty list. What is the correct way to define a default empty list for a query parameter to avoid unexpected behavior?

FastAPI

from fastapi import FastAPI, Query

app = FastAPI()

@app.get("/tags")
async def get_tags(tags: list[str] = []):
    return {"tags": tags}

A Use Optional type hint: tags: Optional[list[str]] = []

B Set default to None and check inside function: tags: list[str] = None

C Keep the default as an empty list directly: tags: list[str] = []

D Use Query with default_factory: tags: list[str] = Query(default_factory=list)

💡 Hint

Mutable default arguments can cause shared state issues in Python functions.

Using a mutable default argument like [] directly can cause all calls to share the same list, leading to bugs. The recommended FastAPI way is to use Query(default_factory=list) to create a new list for each request.