file change monitor for Django

monitor.py

import os
import sys
import time
import signal
import threading
import atexit
import Queue

_interval = 1.0
_times = {}
_files = []

_running = False
_queue = Queue.Queue()
_lock = threading.Lock()

def _restart(path):
    _queue.put(True)
    prefix = 'monitor (pid=%d:):' % os.getpid()
    print >> sys.stderr, '%s Change detected to \'%s\'.' % (prefix, path)
    print >> sys.stderr, '%s Triggering process restart.' % prefix
    os.kill(os.getpid(), signal.SIGINT)

def _modified(path):
    try:
        # If path doesn't denote a file and were previously
        # tracking it, then it has been removed or the file type
        # has changed so force a restart.  If not previously
        # tracking the file then we can ignore it as probably
        # pseudo reference such as when file extracted from a
        # collection of modules contained in a zip file.

        if not os.path.isfile(path):
            return path in _times

        # Check for when file last modified.

        mtime = os.stat(path).st_mtime
        if path not in _times:
            _times[path] = mtime

        # Force restart when modification time has changed, even
        # if time now older, as that could indicate older file
        # has been restored

        if mtime != _times[path]:
            return True
    except:
        # If any exception occured likely that file has been
        # removed just before stat(), so force a restart.
        return True
    return False

def _monitor():
    while 1:
        # Check modification times on all files in sys.modules

        for module in sys.modules.values():
            if not hasattr(module, '__file__'):
                continue
            path = getattr(module, '__file__')
            if not path:
                continue
            if os.path.splitext(path)[1] in ['.pyc', '.pyo', '.pyd']:
                path = path[:-1]
            if _modified(path):
                return _restart(path)

        # Check modification times on files which have
        # specifically be registered for monitoring

        for path in _files:
            if _modified(path):
                return _restart(path)

        # Go to sleep for specified interval.

        try:
            return _queue.get(timeout = _interval)
        except:
            pass

_thread = threading.Thread(target = _monitor)
_thread.setDaemon(True)

def _exiting():
    try:
        _queue.put(True)
    except:
        pass
    _thread.join()

atexit.register(_exiting)

def track(path):
    if not path in _files:
        _files.append(path)

def start(interval = 1.0):
    global _interval
    if interval < _interval:
        _interval = interval

    global _running
    _lock.acquire()
    if not _running:
        prefix = 'monitor (pid=%d):' % os.getpid()
        print >> sys.stderr, '%s Starting change monitor.' % prefix
        _running = True
        _thread.start()
    _lock.release()

This tool works great for so many years now, but after upgrade to Python3 we got this error:

[Thu Jun 10 11:11:07.392991 2021] [wsgi:error] [pid 1453] Exception in thread Thread-1:
[Thu Jun 10 11:11:07.393277 2021] [wsgi:error] [pid 1453] Traceback (most recent call last):
[Thu Jun 10 11:11:07.393310 2021] [wsgi:error] [pid 1453]   File "/usr/lib64/python3.6/threading.py", line 916, in _bootstrap_inner
[Thu Jun 10 11:11:07.393378 2021] [wsgi:error] [pid 1453]     self.run()
[Thu Jun 10 11:11:07.393390 2021] [wsgi:error] [pid 1453]   File "/usr/lib64/python3.6/threading.py", line 864, in run
[Thu Jun 10 11:11:07.393398 2021] [wsgi:error] [pid 1453]     self._target(*self._args, **self._kwargs)
[Thu Jun 10 11:11:07.393519 2021] [wsgi:error] [pid 1453]   File "/home/......../monitor.py", line 61, in _monitor
[Thu Jun 10 11:11:07.393603 2021] [wsgi:error] [pid 1453]     for module in sys.modules.values():
[Thu Jun 10 11:11:07.393615 2021] [wsgi:error] [pid 1453] RuntimeError: dictionary changed size during iteration

I don't know why the dict changes while iteration, but it can be fixed with: for module in sys.modules.copy().values():