How to make a logging handler log to flask-socketio

Question

I want to write a record created by a logging handler to a webpage by passing that record through a Flask SocketIO socket (i.e. print the same log message I get at the console to a webpage).

I see many handlers are available, but none of them appear to readily suit this case. Maybe I need a custom handler?

class MessagePaneHandler:
    # some log data is received 
    socketio.emit("event_name", log_message_from_handler, namespace="/some_namespace", broadcast=True)

Update:

As far as I can tell, the logging.handlers.SocketHandler writes to a raw TCP socket. Can I use a logging.handlers.SocketHandler to write to a Flask SocketIO socket instead? If so, how?

If it helps, this is how I set up my socket on the front end:

var socket = io.connect("http://" + document.domain + ":" + location.port + "/some_namespace");

SocketHandler is not what you want, that writes to a standard networking socket, which is much more lower lever than Socket.IO. I think you'll need to write your own logging handler, I'm not aware of any that sends logs via Socket.IO. — Miguel Grinberg
– Miguel Grinberg, Commented Aug 22, 2020 at 18:30
@MiguelGrinberg is writing an own socket handler if someone wants to write to Flask SocketIO still the way to go? — user7063051
– user7063051, Commented May 31, 2023 at 15:25

Salvatore · Accepted Answer · 2020-08-25 17:52:23Z

2

As far as I could find, none of the built in handlers do this. This custom handler did the trick for me:


from flask_socketio import SocketIO
socketio = SocketIO()

class SocketIOHandler(logging.Handler):
    """A logging handler that emits records with SocketIO."""

    def __init__(self):
        super().__init__()

    def emit(self, record):
        asctime = record.__dict__["asctime"]
        msecs = record.__dict__["msecs"]
        levelname = record.__dict__["levelname"]
        module = record.__dict__["module"]
        message = record.__dict__["message"]
        msg = f"{asctime}.{round(msecs):03} [{levelname:<5.5s}]  {module:<18s}  {message}"
        socketio.emit("update_message_box", msg, namespace="/printing", broadcast=True)

answered Aug 25, 2020 at 17:52

Salvatore

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2 Comments

Rein Over a year ago

This is fantastic! I'm not sure whether this ties into some version of Python or whatever, but I could just access record.asctime, record.levelname etc., from the LogRecord instance attributes. Also: it probably makes sense to return early if socketio.server is None. Other than that I have only gratitude for your implementation.

Intrastellar Explorer Over a year ago

Note: this only works if socketio.emit is synchronous

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