WSGI middleware setting up a queue log handler

lsmw.py

import logging
import sys
from logging.handlers import QueueHandler, QueueListener
from queue import Queue
from time import sleep


class FlushingHandler(QueueHandler):
    def flush(self):
        super().flush()
        pending = self.queue.qsize()
        lastprint = None
        while pending > 0:
            if lastprint != pending:
                print("Flushing %d pending log records..." % pending, file=sys.stderr)
                lastprint = pending
            sleep(0.2)
            pending = self.queue.qsize()
        self.queue.join()


class LogMiddleware:
    def __init__(self, app):
        self.app = app
        self.queue = Queue(-1)
        self.handler = FlushingHandler(self.queue)
        self.log_handler = logging.StreamHandler()
        self.listener = QueueListener(self.queue, self.log_handler)
        self.started = False
        self.prepared = False

    def prepare(self):
        if self.prepared:
            return
        logging.getLogger().addHandler(self.handler)
        self.prepared = True

    def __call__(self, environ, start_response):
        self.prepare()
        if not self.started:
            print("starting...")
            self.listener.start()
            self.started = True
        environ["LOG_MIDDLEWARE_HANDLER"] = self.handler
        return self.app(environ, start_response)