FastAPI Swagger Documentation Showing Incorrect Response Status?

The route implemented via FastAPI (code snippet below) is working fine.

@app.post(path='/user/create/')
async def create_user(response: Response,
                      email: str = Form(),
                      password: str = Form()) -> None:
    """
    Create a new user with this route.
    Possible responses:
    201: New User Created
    400: Bad email id supplied.
    409: User already signed up, hence causing conflict with existing id.
    """
    logging.info(msg=f'New user sign up request from {email}.')
    try:
        user: WebUser = WebUser(email=email, password=password)
        # Either 409 or 201
        response.status_code = [CONFLICT, CREATED][await user.create_new()]
    except ValidationError:
        logging.error(
            msg=f'User creation for {email} failed because of bad request.')
        response.status_code = BAD_REQUEST  # 400

But the FastAPI swagger documentation is showing incorrect (actually totally random) responses. Here is the screenshot.

So, it is not showing 409, and 400, but somehow showing 422 as a possible response. Also, the successful response is actually 201.

So how does FastAPI form these documentations behind the scene and am I doing something wrong to mislead it? Or is it a bug within FastAPI?

This is documented in the Additional Status Codes and Additional Responses in OpenAPI sections of the documentation.

If we rewrite your code like this:

import random
import logging
import pydantic
from fastapi import FastAPI, Response, Form
from fastapi.responses import JSONResponse

app = FastAPI()


class BaseModel(pydantic.BaseModel):
    pass


class Message(BaseModel):
    message: str


class WebUser(BaseModel):
    email: str
    password: str


def user_create_new(user: WebUser):
    if random.randint(0, 100) > 50:
        pass
    else:
        raise ValueError()


@app.post(
    path="/user/create",
    responses={400: {"model": Message}, 409: {"model": Message}},
)
async def create_user(
    response: Response, email: str = Form(), password: str = Form()
) -> None:
    logging.info(msg=f"New user sign up request from {email}.")
    user: WebUser = WebUser(email=email, password=password)
    try:
        user_create_new(user)
        return Message(message=f"Created user {email}")
    except ValueError:
        return JSONResponse(
            status_code=400, content=Message(message="Unable to create user").dict()
        )

Then http://localhost/openapi gets us: