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

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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.