Status code control in FastAPI - Step-by-Step Execution

Concept Flow - Status code control

Receive HTTP request

↓

Call FastAPI endpoint function

↓

Process logic inside function

↓

Decide status code to return

↓

Return response with status code

↓

Client receives response with status code

This flow shows how FastAPI handles an HTTP request, processes it, sets a status code, and sends the response back.

Execution Sample

FastAPI

from fastapi import FastAPI, status

app = FastAPI()

@app.post("/items", status_code=status.HTTP_201_CREATED)
async def create_item():
    return {"message": "Item created"}

This code defines a POST endpoint that returns a 201 Created status code with a JSON message.

Execution Table

Step	Action	Status Code Set	Response Body	Client Receives
1	Client sends POST request to /items	None yet	None yet	Request sent
2	FastAPI calls create_item function	None yet	None yet	Waiting response
3	Function returns {'message': 'Item created'}	201 Created	{"message": "Item created"}	Preparing response
4	FastAPI sends response with status 201	201 Created	{"message": "Item created"}	Response received with status 201
5	Client processes response	201 Created	{"message": "Item created"}	Client sees success message

💡 Response sent with status code 201 Created, ending request cycle

Variable Tracker

Variable	Start	After Step 3	Final
status_code	None	201	201
response_body	None	{"message": "Item created"}	{"message": "Item created"}

Key Moments - 2 Insights

Why do we specify status_code in the decorator instead of returning it inside the function?

Specifying status_code in the decorator sets the HTTP status for the response automatically, as shown in execution_table step 3, so you don't have to manually create a Response object.

What happens if we don't specify a status code in FastAPI?

FastAPI uses the default 200 OK status code, as no status_code is set before step 3 in the execution_table, so the client receives 200 unless changed.

Visual Quiz - 3 Questions

Look at the execution_table, what status code is set when the function returns the response?

A 200 OK

B 201 Created

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check the 'Status Code Set' column at step 3 in the execution_table.

At which step does the client receive the response with the status code?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Client Receives' column to find when the response is received.

If we remove the status_code parameter from the decorator, what status code will the client receive?

A 200 OK

B 404 Not Found

C 201 Created

D 500 Internal Server Error

💡 Hint

Refer to key_moments where default status code behavior is explained.

Concept Snapshot

FastAPI lets you control HTTP status codes by setting status_code in the route decorator.
This sets the response status automatically.
If not set, default is 200 OK.
Use status module constants like status.HTTP_201_CREATED for clarity.
The client receives the status code with the response.
This helps communicate success or errors clearly.

Full Transcript

In FastAPI, when a client sends a request, the framework calls the endpoint function. Inside this function, you can return data. To control the HTTP status code sent back, you specify status_code in the route decorator. For example, setting status_code=status.HTTP_201_CREATED means the client receives a 201 Created status. If you don't specify it, FastAPI sends 200 OK by default. The execution table shows each step: request sent, function called, response prepared with status code, response sent, and client receives it. Variables like status_code and response_body change during execution. Beginners often wonder why status_code is set in the decorator and what happens if omitted. The answer is that the decorator sets the status automatically, and default is 200 if not set. This control helps clients understand if their request succeeded or failed.

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