Logging using multiple process to one file in python

Question

I am trying to setup logging between multiple processes using the QueueHandler. I am seeing the same log in the log file printed multiple times. Using this as a template (https://docs.python.org/3/howto/logging-cookbook.html#logging-to-a-single-file-from-multiple-processes)

EDIT

multi processing file:

import logging
from logging.handlers   import RotatingFileHandler, QueueHandler
from multiprocessing import Process
from queue import Empty

class MultiProcessQueueLoggingListner(Process):
    def __init__(self, name, queue):
        super().__init__()
        self.name = name
        self.queue = queue
        self.logger = logging.getLogger(name)
        self.file_handler = RotatingFileHandler(name, maxBytes=536870912, backupCount=2)
        self.formatter = logging.Formatter('%(asctime)s %(processName)-10s %(name)s %(levelname)-8s %(message)s')
        self.file_handler.setFormatter(self.formatter)
        self.logger.addHandler(self.file_handler)

    def run(self):
        while True:
            try:
                record = self.queue.get()
                if record is None:
                    break
                self.logger.handle(record)
            except Exception:
                import sys, traceback
                print('Whoops! Problem:', file=sys.stderr)
                traceback.print_exc(file=sys.stderr)


class MulitProcessQueueLogger(object):
def __init__(self, name, queue):
    self.name = name
    self.queue = queue
    self.queue_handler = QueueHandler(queue)
    self.logger = logging.getLogger(name)
    self.logger.addHandler(self.queue_handler)
    self.logger.setLevel(logging.DEBUG)

test file:

import multi_process_logging
import multiprocessing
from time import sleep


def worker(po):
    name = multiprocessing.current_process().name
    po = multi_process_logging.MulitProcessQueueLogger('test.log', q)
    print("In worker")
    for i in range(10):
        po.logger.info(f"Logging from {name} line {i}")
    po.queue.put(None)

def main():
    q = multiprocessing.Queue()
    lp = multi_process_logging.MultiProcessQueueLoggingListner('test.log', q)
    lp.start()
    p = multiprocessing.Process(target=worker, args=(q,))
    p.start()
    p.join()
    lp.join()



if __name__ == '__main__':
    main()

The issue I see is that the test.log file contains multiple lines for the same entry. The program stops now and doesn't run indefinite but still seeing multiple lines

    cat test.log | grep 'line 0'
2018-09-26 16:32:40,117 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:40,117 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:40,117 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:40,117 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:50,318 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:50,318 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:50,318 Process-2  test.log INFO     Logging from Process-2 line 0
2018-09-26 16:32:50,318 Process-2  test.log INFO     Logging from Process-2 line 0

I removed test.log before the run to rule out the append onto an existing log file, but still seeing multiple logs.

Thanks

Answer 1

Your problem is caused by the fact that you're checking for a None to break out of the loop, but that will never come because a QueueHandler always writes a LogRecord to a queue, never None. If you want to write None to the queue, you need to write it directly rather than by doing po.logger.info(None). For example, store the queue as an attribute named queue of your MulitProcessQueueLogger instance and then do po.queue.put(None) in worker().