status_code

from fastapi import FastAPI
from starlette import status   # Import status code constants

app = FastAPI()

@app.post(
    "/items/",
    status_code=status.HTTP_201_CREATED,   # Specify the status code to return on success
)
def 関数名():
    # Processing logic
    return {"message": "作成しました"}

The following example shows how to assign appropriate status codes to each HTTP method using CRUD operations on an item resource.

# ---- main.py ----

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
from starlette import status

# Create a FastAPI instance
app = FastAPI()


# Define the request body schema
class ItemCreate(BaseModel):
    name: str
    price: float


class ItemUpdate(BaseModel):
    name: str
    price: float


# Sample data store (use a real database in production)
items: dict = {
    1: {"id": 1, "name": "りんご", "price": 150.0},
    2: {"id": 2, "name": "みかん", "price": 80.0},
}
next_id: int = 3


# GET: Returns a list of all items
# Returns 200 OK on success (this is the default, so it can be omitted)
@app.get("/items/", status_code=status.HTTP_200_OK)
def get_items():
    return list(items.values())


# GET: Returns the item with the specified ID
# Raises an HTTPException with 404 if the item is not found
@app.get("/items/{item_id}", status_code=status.HTTP_200_OK)
def get_item(item_id: int):
    if item_id not in items:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="商品が見つかりません")
    return items[item_id]


# POST: Creates a new item
# Returns 201 Created on success
@app.post("/items/", status_code=status.HTTP_201_CREATED)
def create_item(item: ItemCreate):
    global next_id
    new_item = {"id": next_id, "name": item.name, "price": item.price}
    items[next_id] = new_item
    next_id += 1
    return new_item


# PUT: Updates an existing item
# Returns 200 OK on success
@app.put("/items/{item_id}", status_code=status.HTTP_200_OK)
def update_item(item_id: int, item: ItemUpdate):
    if item_id not in items:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="商品が見つかりません")
    items[item_id] = {"id": item_id, "name": item.name, "price": item.price}
    return items[item_id]


# DELETE: Deletes the item with the specified ID
# Returns 204 No Content on success (no response body is returned)
@app.delete("/items/{item_id}", status_code=status.HTTP_204_NO_CONTENT)
def delete_item(item_id: int):
    if item_id not in items:
        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="商品が見つかりません")
    del items[item_id]
    # Return None because 204 responses must not include a response body
    return None

status_code is a parameter passed to the decorator that sets the HTTP status code returned to the client when an endpoint completes successfully. You can specify a numeric value directly, but using constants from the starlette.status module (such as HTTP_201_CREATED) makes the intent clearer. In RESTful API design, it is conventional to return 200 for GET, 201 for resource creation with POST, and 204 for DELETE. Note that status_code is only the default value — if HTTPException is raised inside an endpoint, it overrides the specified code. For endpoints that specify 204, FastAPI automatically removes the response body, so it is recommended to use return None or a bare return.