Custom error response models in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct FastAPI class.

FastAPI

from fastapi import [1]
app = [1]()

Drag options to blanks, or click blank then click option'

AFastAPI

BRequest

CResponse

DDepends

Attempts:

3 left

💡 Hint

Common Mistakes

Importing Request or Response instead of FastAPI.

Trying to create app with a wrong class.

You need to import FastAPI to create the app instance.

2fill in blank

medium

Complete the code to define a Pydantic model for error response with a message field.

FastAPI

from pydantic import BaseModel

class ErrorResponse([1]):
    message: str

Drag options to blanks, or click blank then click option'

ABaseModel

BModel

CErrorModel

DResponseModel

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong base class like Model or ResponseModel.

Forgetting to inherit from any class.

Pydantic models inherit from BaseModel to define data schemas.

3fill in blank

hard

Fix the error in the route decorator to specify the response model for errors.

FastAPI

@app.get("/items/{item_id}", responses={404: {"model": [1])
async def read_item(item_id: int):
    return {"item_id": item_id}

Drag options to blanks, or click blank then click option'

AItem

BErrorResponse

CHTTPException

DResponse

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong model name like Item or Response.

Using an instance instead of the class.

The responses dictionary uses the error response model class name to describe the error schema.

4fill in blank

hard

Fill both blanks to raise an HTTPException with a custom error response model.

FastAPI

from fastapi import HTTPException

if not item:
    raise HTTPException(status_code=[1], detail=[2])

Drag options to blanks, or click blank then click option'

A404

B"Item not found"

C"Invalid request"

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong status codes like 500 for not found.

Passing non-string detail values.

Use status code 404 for not found and a clear message string for detail.

5fill in blank

hard

Fill all three blanks to define a route that returns a custom error response model on failure.

FastAPI

from fastapi import status

@app.get("/users/{user_id}", responses=[1]: {"model": [2])
async def get_user(user_id: int):
    if user_id != 1:
        raise HTTPException(status_code=[3], detail="User not found")
    return {"user_id": user_id}

Drag options to blanks, or click blank then click option'

Astatus.HTTP_404_NOT_FOUND

BErrorResponse

C404

Dstatus.HTTP_400_BAD_REQUEST

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing status codes like 400 and 404.

Using string '404' instead of status constants.

Use the status code constant for 404, the error model class, and the same status code for the exception.

Practice

1. What is the main purpose of using custom error response models in FastAPI?

easy

A. To define a clear and consistent structure for error messages returned by the API

B. To speed up the API response time

C. To automatically fix errors in the API code

D. To hide all error messages from the client

Solution

1. Step 1: Understand error response models

   Custom error response models define how error messages look, making them clear and consistent.

2. Step 2: Identify the purpose in FastAPI

   FastAPI uses these models to send structured error info to clients, improving API usability.

3. Final Answer:

   To define a clear and consistent structure for error messages returned by the API -> Option A

4. Quick Check:

   Custom error models = clear error messages [OK]

Hint: Custom error models shape error messages clearly [OK]

Common Mistakes:

• Thinking they speed up API responses
• Believing they fix code errors automatically
• Assuming they hide errors completely

2. Which of the following is the correct way to define a custom error response model using Pydantic in FastAPI?

easy

A. class ErrorResponse: message = str; code = int

B. class ErrorResponse(BaseModel): message: str; code: int

C. ErrorResponse = {'message': str, 'code': int}

D. def ErrorResponse(message: str, code: int): return {'message': message, 'code': code}

Solution

1. Step 1: Recognize Pydantic model syntax

   Pydantic models are defined as classes inheriting from BaseModel with typed attributes.

2. Step 2: Match correct syntax

   class ErrorResponse(BaseModel): message: str; code: int correctly defines a class with typed fields using BaseModel.

3. Final Answer:

   class ErrorResponse(BaseModel): message: str; code: int -> Option B

4. Quick Check:

   Pydantic model = class with BaseModel [OK]

Hint: Use class with BaseModel and typed fields [OK]

Common Mistakes:

• Defining models as functions
• Using plain dictionaries instead of classes
• Missing BaseModel inheritance

3. Given this FastAPI code snippet, what will be the response when a ValueError is raised?

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel

app = FastAPI()

class ErrorResponse(BaseModel):
    detail: str

@app.exception_handler(ValueError)
async def value_error_handler(request: Request, exc: ValueError):
    return JSONResponse(
        status_code=400,
        content=ErrorResponse(detail=str(exc)).dict()
    )

@app.get("/test")
async def test():
    raise ValueError("Invalid input")

medium

A. Empty response with status 204

B. Plain text 'Invalid input' with status 500

C. {"detail": "Invalid input"} with status 400

D. JSON with key 'error' and message 'Invalid input' with status 400

Solution

1. Step 1: Understand exception handler behavior

   The handler catches ValueError and returns JSONResponse with ErrorResponse model content and status 400.

2. Step 2: Check response content and status

   The content is the dict form of ErrorResponse with detail set to the exception message, so JSON has key 'detail' with 'Invalid input'.

3. Final Answer:

   {"detail": "Invalid input"} with status 400 -> Option C

4. Quick Check:

   Exception handler returns JSON with detail key [OK]

Hint: Exception handler returns model dict as JSON with status [OK]

Common Mistakes:

• Expecting plain text instead of JSON
• Confusing status codes
• Assuming different JSON key names

4. Identify the error in this FastAPI custom error handler code:

from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from pydantic import BaseModel

app = FastAPI()

class ErrorResponse(BaseModel):
    message: str

@app.exception_handler(KeyError)
async def key_error_handler(request: Request, exc: KeyError):
    return JSONResponse(
        status_code=404,
        content=ErrorResponse(message=exc).dict()
    )

medium

A. Missing BaseModel inheritance in ErrorResponse

B. Exception handler must be synchronous, not async

C. Using status code 404 instead of 400

D. Passing the exception object directly instead of converting to string

Solution

1. Step 1: Check how exception is passed to model

   The code passes exc (KeyError object) directly to message field which expects a string.

2. Step 2: Identify correct usage

   It should convert exc to string with str(exc) to avoid type errors.

3. Final Answer:

   Passing the exception object directly instead of converting to string -> Option D

4. Quick Check:

   Exception message must be string [OK]

Hint: Convert exception to string before passing to model [OK]

Common Mistakes:

• Passing exception object without str()
• Confusing async/sync handler rules
• Incorrect status code choice

5. You want to create a FastAPI app that returns a custom error response with fields error_code (int) and error_msg (str) whenever a RuntimeError occurs. Which of the following code snippets correctly implements this behavior?

hard

A. class CustomError(BaseModel): error_code: int; error_msg: str

@app.exception_handler(RuntimeError)
async def runtime_error_handler(request: Request, exc: RuntimeError):
return JSONResponse(status_code=500, content=CustomError(error_code=1001, error_msg=str(exc)).dict())

B. class CustomError(BaseModel): error_code: str; error_msg: int

@app.exception_handler(RuntimeError)
async def runtime_error_handler(request: Request, exc: RuntimeError):
return JSONResponse(status_code=400, content=CustomError(error_code='1001', error_msg=exc).dict())

C. class CustomError:
def __init__(self, error_code, error_msg):
self.error_code = error_code
self.error_msg = error_msg

@app.exception_handler(RuntimeError)
async def runtime_error_handler(request: Request, exc: RuntimeError):
return JSONResponse(status_code=500, content=CustomError(1001, str(exc)))

D. @app.exception_handler(RuntimeError)
async def runtime_error_handler(request: Request, exc: RuntimeError):
return JSONResponse(status_code=500, content={'error_code': 1001, 'error_msg': str(exc)})

Solution

1. Step 1: Define correct Pydantic model

   class CustomError(BaseModel): error_code: int; error_msg: str @app.exception_handler(RuntimeError) async def runtime_error_handler(request: Request, exc: RuntimeError): return JSONResponse(status_code=500, content=CustomError(error_code=1001, error_msg=str(exc)).dict()) defines CustomError inheriting from BaseModel with correct field types (int and str).

2. Step 2: Implement exception handler properly

   class CustomError(BaseModel): error_code: int; error_msg: str @app.exception_handler(RuntimeError) async def runtime_error_handler(request: Request, exc: RuntimeError): return JSONResponse(status_code=500, content=CustomError(error_code=1001, error_msg=str(exc)).dict())'s handler returns JSONResponse with status 500 and content as dict from CustomError instance, converting exception to string.

3. Step 3: Check other options for errors

   class CustomError(BaseModel): error_code: str; error_msg: int @app.exception_handler(RuntimeError) async def runtime_error_handler(request: Request, exc: RuntimeError): return JSONResponse(status_code=400, content=CustomError(error_code='1001', error_msg=exc).dict()) swaps types incorrectly and passes exc without str(); class CustomError: def __init__(self, error_code, error_msg): self.error_code = error_code self.error_msg = error_msg @app.exception_handler(RuntimeError) async def runtime_error_handler(request: Request, exc: RuntimeError): return JSONResponse(status_code=500, content=CustomError(1001, str(exc))) uses plain class not BaseModel and returns object not dict; @app.exception_handler(RuntimeError) async def runtime_error_handler(request: Request, exc: RuntimeError): return JSONResponse(status_code=500, content={'error_code': 1001, 'error_msg': str(exc)}) skips model usage.

4. Final Answer:

   Option A correctly defines model and handler returning proper JSON response -> Option A

5. Quick Check:

   Use BaseModel with typed fields and dict() in handler [OK]

Hint: Use BaseModel and dict() for error response content [OK]

Common Mistakes:

• Swapping field types in model
• Not converting exception to string
• Returning model instance instead of dict
• Skipping Pydantic model for error response