How do you control HTTP status codes and return custom responses in FastAPI?

FastAPI lets you set the default response status code on the decorator, raise HTTPException for errors, and return Response subclasses for full control over headers and body.

from fastapi import FastAPI, HTTPException, status
from fastapi.responses import JSONResponse, Response, FileResponse

app = FastAPI()

fake_db = {1: {"name": "Foo"}, 2: {"name": "Bar"}}

# Set default success status code
@app.post("/items", status_code=status.HTTP_201_CREATED)
def create_item(name: str):
    return {"name": name}

# Raise HTTP errors
@app.get("/items/{item_id}")
def get_item(item_id: int):
    if item_id not in fake_db:
        raise HTTPException(
            status_code=status.HTTP_404_NOT_FOUND,
            detail=f"Item {item_id} not found",
            headers={"X-Error": "Item missing"},  # custom header on error
        )
    return fake_db[item_id]

# Custom JSONResponse for full control
@app.get("/custom")
def custom_response():
    return JSONResponse(
        status_code=200,
        content={"message": "ok"},
        headers={"X-Custom": "value"},
    )

# 204 No Content (no response body)
@app.delete("/items/{item_id}", status_code=status.HTTP_204_NO_CONTENT)
def delete_item(item_id: int):
    fake_db.pop(item_id, None)
    return Response(status_code=204)

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