How to check if a logger exists

Question

I’ve had to extend the existing logging library to add some features. One of these is a feature which lets a handler listen to any existing log without knowing if it exists beforehand. The following allows the handler to listen, but does not check if the log exists:

def listen_to_log(target, handler):
    logging.getLogger(target).addHandler(handler)

The issue is that I don’t want any handler to listen to a log which isn’t being logged to, and want to raise a ValueError if the log doesn’t already exist. Ideally I’d do something like the following:

def listen_to_log(target, handler):
    if not logging.logExists(target):
        raise ValueError('Log not found')
    logging.getLogger(target).addHandler(handler)

So far I have been unable to find a function like logging.logExists, is there such a function, or a convenient workaround?

Asked By: shayaan

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Answer 1

Of course, if you attach a handler to the root logger, you will (generally) get all messages (unless a library author specifically chooses not to propagate to root handlers).

Why should you care if a handler listens to a log which hasn’t been created yet? You can apply filters to a handler that ignore certain events.

This doesn’t specifically answer your question, because I see your question as an instance of an XY Problem.

Answered By: Vinay Sajip

Answer 2

WARNING. This is not documented. It may change without notice.

The logging module internally uses a single Manager object to hold the hierarchy of loggers in a dict accessible as:

import logging
logging.Logger.manager.loggerDict

All loggers except the root logger are stored in that dict under their names.

There are few examples on the net:
http://code.activestate.com/lists/python-list/621740/
and
https://michaelgoerz.net/notes/use-of-the-logging-module-in-python.html (uses Python 2 print)

Answered By: VPfB

Answer 3

Workaround that works for me and possibly you:

When you create a logger for your own code, you will almost certainly create a logger with handlers (file handler and/or console handler).
When you have not yet created a logger and you get the ‘root’ logger by

logger = logging.getLogger()

then this logger will have no handlers yet.
Therefore, I always check whether the logger above results in a logger that has handlers by

logging.getLogger().hasHandlers()

So I create a logger only if the root logger does not have handlers:

logger = <create_my_logger> if not logging.getLogger().hasHandlers() else logging.getLogger()

The “create_my_logger” snippet represents my code/function that returns a logger (with handlers).

Answered By: Max Quant

How to check if a logger exists

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