Exceptions - HTTPException and WebSocketException

These are the exceptions that you can raise to show errors to the client.

When you raise an exception, as would happen with normal Python, the rest of the excecution is aborted. This way you can raise these exceptions from anywhere in the code to abort a request and show the error to the client.

You can use:

• HTTPException
• WebSocketException

These exceptions can be imported directly from fastapi:

from fastapi import HTTPException, WebSocketException

fastapi.HTTPException

HTTPException(status_code, detail=None, headers=None)

Bases: HTTPException

An HTTP exception you can raise in your own code to show errors to the client.

This is for client errors, invalid authentication, invalid data, etc. Not for server errors in your code.

Read more about it in the FastAPI docs for Handling Errors.

Example

from fastapi import FastAPI, HTTPException

app = FastAPI()

items = {"foo": "The Foo Wrestlers"}


@app.get("/items/{item_id}")
async def read_item(item_id: str):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="Item not found")
    return {"item": items[item_id]}

PARAMETER	DESCRIPTION
status_code	HTTP status code to send to the client. TYPE: int
detail	Any data to be sent to the client in the detail key of the JSON response. TYPE: Any DEFAULT: None
headers	Any headers to send to the client in the response. TYPE: Optional[Dict[str, str]] DEFAULT: None

Source code in fastapi/exceptions.py

def __init__(
    self,
    status_code: Annotated[
        int,
        Doc(
            """
            HTTP status code to send to the client.
            """
        ),
    ],
    detail: Annotated[
        Any,
        Doc(
            """
            Any data to be sent to the client in the `detail` key of the JSON
            response.
            """
        ),
    ] = None,
    headers: Annotated[
        Optional[Dict[str, str]],
        Doc(
            """
            Any headers to send to the client in the response.
            """
        ),
    ] = None,
) -> None:
    super().__init__(status_code=status_code, detail=detail, headers=headers)

status_code instance-attribute

status_code = status_code

detail instance-attribute

detail = detail

headers instance-attribute

headers = headers

fastapi.WebSocketException

WebSocketException(code, reason=None)

Bases: WebSocketException

A WebSocket exception you can raise in your own code to show errors to the client.

This is for client errors, invalid authentication, invalid data, etc. Not for server errors in your code.

Read more about it in the FastAPI docs for WebSockets.

Example

from typing import Annotated

from fastapi import (
    Cookie,
    FastAPI,
    WebSocket,
    WebSocketException,
    status,
)

app = FastAPI()

@app.websocket("/items/{item_id}/ws")
async def websocket_endpoint(
    *,
    websocket: WebSocket,
    session: Annotated[str | None, Cookie()] = None,
    item_id: str,
):
    if session is None:
        raise WebSocketException(code=status.WS_1008_POLICY_VIOLATION)
    await websocket.accept()
    while True:
        data = await websocket.receive_text()
        await websocket.send_text(f"Session cookie is: {session}")
        await websocket.send_text(f"Message text was: {data}, for item ID: {item_id}")

PARAMETER	DESCRIPTION
code	A closing code from the valid codes defined in the specification. TYPE: int
reason	The reason to close the WebSocket connection. It is UTF-8-encoded data. The interpretation of the reason is up to the application, it is not specified by the WebSocket specification. It could contain text that could be human-readable or interpretable by the client code, etc. TYPE: Union[str, None] DEFAULT: None

Source code in fastapi/exceptions.py

def __init__(
    self,
    code: Annotated[
        int,
        Doc(
            """
            A closing code from the
            [valid codes defined in the specification](https://datatracker.ietf.org/doc/html/rfc6455#section-7.4.1).
            """
        ),
    ],
    reason: Annotated[
        Union[str, None],
        Doc(
            """
            The reason to close the WebSocket connection.

            It is UTF-8-encoded data. The interpretation of the reason is up to the
            application, it is not specified by the WebSocket specification.

            It could contain text that could be human-readable or interpretable
            by the client code, etc.
            """
        ),
    ] = None,
) -> None:
    super().__init__(code=code, reason=reason)

code instance-attribute

code = code

reason instance-attribute

reason = reason or ''