Status code control in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What status code does this FastAPI endpoint return?

Consider this FastAPI endpoint code. What HTTP status code will the client receive when calling /items/42?

FastAPI

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get('/items/{item_id}')
async def read_item(item_id: int):
    if item_id == 42:
        return JSONResponse(content={'item_id': item_id}, status_code=202)
    return {'item_id': item_id}

A 200

B 500

C 404

D 202

💡 Hint

Look at the status_code parameter in JSONResponse for item_id 42.

When item_id is 42, the endpoint returns a JSONResponse with status_code=202. Otherwise, it returns a default 200 response.

📝 Syntax

intermediate

Which option correctly sets a 201 status code for a POST endpoint?

You want to create a FastAPI POST endpoint that returns status code 201 when a new item is created. Which code snippet correctly sets this status code?

A

@app.post('/items')
async def create_item(item: dict):
    return Response(content=item, status_code=201)

B

@app.post('/items')
async def create_item(item: dict):
    return JSONResponse(content=item, status_code=201)

C

@app.post('/items', status_code=201)
async def create_item(item: dict):
    return item

D

@app.post('/items')
async def create_item(item: dict):
    return {'item': item, 'status': 201}

💡 Hint

Check how FastAPI allows setting status codes directly in the decorator.

Option C uses the status_code parameter in the decorator, which is the recommended way to set the response status code for simple returns.

🔧 Debug

advanced

Why does this FastAPI endpoint always return status 200 instead of 404?

Examine the code below. The intention is to return 404 if the item is not found. Why does it always return 200?

FastAPI

from fastapi import FastAPI, HTTPException

app = FastAPI()

items = {1: 'apple', 2: 'banana'}

@app.get('/items/{item_id}')
async def get_item(item_id: int):
    if item_id not in items:
        raise HTTPException(status_code=404, detail='Item not found')
    return {'item': items[item_id]}

A HTTPException must be raised, not returned, to trigger the error response.

B The status_code 404 is invalid and defaults to 200.

C The items dictionary is empty, so the condition never triggers.

D FastAPI does not support HTTPException for status control.

💡 Hint

Think about how exceptions work in Python and FastAPI.

Returning HTTPException just sends it as a normal response body with status 200. You must raise it to trigger the error status.

❓ state_output

advanced

What is the status code and response body when this FastAPI endpoint is called with /users/0?

Analyze the following FastAPI endpoint. What status code and JSON response will the client receive when requesting /users/0?

FastAPI

from fastapi import FastAPI, Response

app = FastAPI()

@app.get('/users/{user_id}')
async def get_user(user_id: int):
    if user_id == 0:
        return Response(content='User not found', status_code=404, media_type='text/plain')
    return {'user_id': user_id}

A Status code 404 with plain text body 'User not found'

B Status code 200 with JSON body {'user_id': 0}

C Status code 404 with JSON body {'detail': 'User not found'}

D Status code 500 with empty body

💡 Hint

Look at the Response object and its parameters.

The endpoint returns a Response with status 404 and plain text content 'User not found' when user_id is 0.

🧠 Conceptual

expert

Which statement about FastAPI status code control is true?

Select the correct statement about how FastAPI handles HTTP status codes in endpoint responses.

A Returning a dictionary from an endpoint always results in status code 500 unless a status_code is set explicitly.

B Setting status_code in the decorator changes the default response status code unless overridden by returning a Response with a different status_code.

C FastAPI automatically sets status code 201 for all POST requests regardless of decorator or return value.

D Raising HTTPException changes the response body but does not affect the HTTP status code sent to the client.

💡 Hint

Think about how decorator status_code and return values interact.

The status_code in the decorator sets the default status code. Returning a Response with a different status_code overrides it. Other options are incorrect.

Practice

1. What is the purpose of setting a status_code in a FastAPI route?

easy

A. To define the data type of the response

B. To change the URL path of the route

C. To tell the client if the request was successful or if there was an error

D. To set the server's port number

Solution

1. Step 1: Understand HTTP status codes

   Status codes communicate the result of a request, like success (200) or error (404).

2. Step 2: Role of status_code in FastAPI

   Setting status_code in a route tells the client what happened with their request.

3. Final Answer:

   To tell the client if the request was successful or if there was an error -> Option C

4. Quick Check:

   Status code = success or error info [OK]

Hint: Status codes show request success or failure [OK]

Common Mistakes:

• Confusing status_code with URL path
• Thinking status_code sets response data type
• Mixing status_code with server settings

2. Which of the following is the correct way to set a 201 status code for a POST route in FastAPI?

easy

A. @app.post('/items')
def create_item():
status_code = 201
return {'message': 'Created'}

B. @app.post('/items')
def create_item():
return JSONResponse({'message': 'Created'}, status=201)

C. @app.post('/items')
def create_item():
return {'message': 'Created'}, 201

D. @app.post('/items', status_code=201)
def create_item():
return {'message': 'Created'}

Solution

1. Step 1: Check FastAPI decorator usage

   FastAPI allows setting status_code directly in the route decorator.

2. Step 2: Analyze each option

   @app.post('/items', status_code=201) def create_item(): return {'message': 'Created'} correctly sets status_code=201 in the decorator. @app.post('/items') def create_item(): status_code = 201 return {'message': 'Created'} wrongly assigns inside function without effect. @app.post('/items') def create_item(): return {'message': 'Created'}, 201 returns a tuple which FastAPI does not accept. @app.post('/items') def create_item(): return JSONResponse({'message': 'Created'}, status=201) uses JSONResponse but with wrong parameter name (status instead of status_code).

3. Final Answer:

   @app.post('/items', status_code=201) def create_item(): return {'message': 'Created'} -> Option D

4. Quick Check:

   Set status_code in decorator = @app.post('/items', status_code=201) def create_item(): return {'message': 'Created'} [OK]

Hint: Use status_code in decorator for simple status setting [OK]

Common Mistakes:

• Trying to set status_code inside function without response object
• Returning tuple with status code (not supported)
• Using wrong parameter name in JSONResponse

3. What status code will this FastAPI route return when called?

from fastapi import FastAPI
from fastapi.responses import JSONResponse

app = FastAPI()

@app.get('/data')
async def get_data():
    return JSONResponse(content={'info': 'ok'}, status_code=202)

medium

A. 202 Accepted

B. 201 Created

C. 200 OK

D. 404 Not Found

Solution

1. Step 1: Identify the response type and status code

   The route returns a JSONResponse with status_code=202.

2. Step 2: Understand HTTP status code 202

   Status code 202 means "Accepted" indicating the request is accepted but not completed yet.

3. Final Answer:

   202 Accepted -> Option A

4. Quick Check:

   JSONResponse with status_code=202 = 202 Accepted [OK]

Hint: Check status_code in JSONResponse for actual returned code [OK]

Common Mistakes:

• Assuming default 200 OK without checking status_code
• Confusing 201 Created with 202 Accepted
• Ignoring status_code parameter in JSONResponse

4. Identify the error in this FastAPI route that tries to return a 404 status code:

from fastapi import FastAPI

app = FastAPI()

@app.get('/item/{item_id}', status_code=404)
async def read_item(item_id: int):
    return {'item_id': item_id}

medium

A. Function must raise HTTPException for 404, not set status_code

B. Setting status_code=404 in decorator always returns 404 even if item exists

C. Route path is invalid syntax

D. Missing return type annotation causes error

Solution

1. Step 1: Understand status_code in decorator

   Setting status_code=404 in the decorator makes every response return 404 status regardless of logic.

2. Step 2: Why this is a problem

   The route returns data but status 404 means "Not Found" which is misleading if item exists.

3. Final Answer:

   Setting status_code=404 in decorator always returns 404 even if item exists -> Option B

4. Quick Check:

   Decorator status_code sets fixed response code [OK]

Hint: Decorator status_code sets fixed code, use exceptions for errors [OK]

Common Mistakes:

• Using status_code=404 in decorator to signal missing item
• Not raising HTTPException for 404 errors
• Assuming status_code in decorator is dynamic

5. You want to create a FastAPI route that returns 201 when a new user is created, but 400 if the username is missing. Which is the best way to control the status codes?

hard

A. Use status_code=201 in decorator and raise HTTPException(status_code=400) if username missing

B. Set status_code=201 in decorator and return JSONResponse with status_code=400 inside function

C. Return a tuple with data and status code 201 or 400

D. Set status_code=400 in decorator and return 201 in response body

Solution

1. Step 1: Understand status code control methods

   Setting status_code=201 in decorator sets default success code. Raising HTTPException sends error with desired status.

2. Step 2: Evaluate options

   Use status_code=201 in decorator and raise HTTPException(status_code=400) if username missing uses decorator for success and exception for error, which is correct. Set status_code=201 in decorator and return JSONResponse with status_code=400 inside function mixes decorator and response status codes causing confusion. Return a tuple with data and status code 201 or 400 returning tuple is invalid in FastAPI. Set status_code=400 in decorator and return 201 in response body sets wrong default status code and misuses response body.

3. Final Answer:

   Use status_code=201 in decorator and raise HTTPException(status_code=400) if username missing -> Option A

4. Quick Check:

   Decorator for success, exceptions for errors [OK]

Hint: Use decorator for success, raise HTTPException for errors [OK]

Common Mistakes:

• Returning tuples with status codes (not supported)
• Setting error status_code in decorator instead of raising exception
• Mixing response status codes inconsistently