Status code control in FastAPI - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of status code control in FastAPI?

Status code control in FastAPI lets you specify the HTTP response code sent back to the client, showing if the request was successful, failed, or something else.

beginner

How do you set a custom status code for a response in a FastAPI path operation?

You can set a custom status code by using the status_code parameter in the path operation decorator, like @app.post('/items/', status_code=201).

beginner

What is the default status code for a successful GET request in FastAPI?

The default status code for a successful GET request in FastAPI is 200 OK.

intermediate

How can you return a different status code dynamically inside a FastAPI path function?

You can return a Response or JSONResponse object with a custom status_code set, or raise an HTTPException with the desired status code.

beginner

What status code should you use in FastAPI when a resource is successfully created?

Use status code 201 Created to indicate a resource was successfully created.

Which status code indicates a successful POST request that created a resource?

A 200

B 201

C 404

D 500

201 means the resource was created successfully.

How do you specify a status code of 204 (No Content) in a FastAPI route?

A Use <code>status_code=204</code> in the decorator

B Return <code>None</code> from the function

C Raise <code>HTTPException(status_code=204)</code>

D Set <code>response.status_code = 204</code> inside the function

You specify it in the decorator with status_code=204.

What happens if you don't specify a status code in a FastAPI GET route?

A FastAPI returns 404 by default

B FastAPI returns 500 by default

C FastAPI returns 200 by default

D FastAPI returns 201 by default

The default status code for successful GET is 200.

Which FastAPI class can you use to raise an error with a specific status code?

A Response

B Request

C JSONResponse

D HTTPException

HTTPException lets you raise errors with custom status codes.

To return a 404 Not Found error in FastAPI, you should:

A Raise <code>HTTPException(status_code=404)</code>

B Return <code>None</code>

C Set <code>status_code=404</code> in the decorator

D Return a string '404'

Raising HTTPException with 404 tells FastAPI to send a 404 response.

Explain how to control HTTP status codes in FastAPI path operations.

Think about decorator options and exceptions.

Describe when and why you would use status code 201 in FastAPI.

Consider POST requests creating new items.

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