Using LogRecordFactory in python to add custom fields for logging

Question

I am trying to add a custom field in my logging using LogRecordFactory. I am repeatedly calling a class and every time I do that, I want to set the custom_attribute in the init module so the remainder of the code within the class will have this attribute. But I cannot get this to work. I found the following which works, but its static.

import logging

old_factory = logging.getLogRecordFactory()

def record_factory(*args, **kwargs):
    record = old_factory(*args, **kwargs)
    record.custom_attribute = "whatever"
    return record



logging.basicConfig(format="%(custom_attribute)s - %(message)s")
logging.setLogRecordFactory(record_factory)
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
logging.debug("test")

This will output correctly:

whatever - test

However, my use case is that the custom_attribute will vary. Every time I call a specific function, I want to change this. So it seems like record_factory needs another parameter passed to it so it can then return the correct record with the new parameter. But I cant figure it out. I have tried adding a parameter to the function, but when I make the call I get:

TypeError: __init__() missing 7 required positional arguments: 'name', 'level', 'pathname', 'lineno', 'msg', 'args', and 'exc_info'

I think this has something to do with the *args and **kwargs but I don't really know. Also, why are there no parenthesis after record_factory when its called by logging.setLogRecordFactory? I have never seen a function work like this.

Answer 1

You can try to use closure:

import logging

old_factory = logging.getLogRecordFactory()

def record_factory_factory(context_id):
    def record_factory(*args, **kwargs):
        record = old_factory(*args, **kwargs)
        record.custom_attribute = context_id
        return record
    return record_factory


logging.basicConfig(format="%(custom_attribute)s - %(message)s")
logging.setLogRecordFactory(record_factory_factory("whatever"))
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
logging.debug("test")

logging.setLogRecordFactory(record_factory_factory("whatever2"))
logger = logging.getLogger()
logger.setLevel(logging.DEBUG)
logging.debug("test")

result:

$ python3 log_test.py                                                          
whatever - test
whatever2 - test