Wake a desktop agent when a file arrives: a watch plus a sweep you can trust

Start a recursive watch on the account folder and push every event onto a queue. Treat what lands on the queue as a hint, not a fact - the real work happens later, in the sweep.

from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler

class Hint(FileSystemEventHandler):
    """Every event is a hint that something may have changed - never a guarantee."""
    def __init__(self, queue):
        self.queue = queue

    def on_created(self, event):
        if not event.is_directory:
            self.queue.put(event.src_path)

    def on_moved(self, event):
        if not event.is_directory:
            self.queue.put(event.dest_path)

def watch(root, queue):
    observer = Observer()
    observer.schedule(Hint(queue), root, recursive=True)   # the whole tree, one watch
    observer.start()
    return observer

On Windows the native layer is ReadDirectoryChangesW, which can watch a subtree in a single call; watchdog wraps it for you and falls back to polling where no native observer exists.

A notification fires on the first bytes, not the last. If you read immediately you can read half a file. Hold off until the size has stopped changing and the file opens without a sharing conflict - the same "wait until the write is finished" move the chokidar library makes by waiting until "the file size has stayed constant."

import os, time

def settled(path, wait=2.0, tries=30):
    """True once the file has stopped growing and opens cleanly; False if still being written."""
    last = -1
    for _ in range(tries):
        try:
            size = os.path.getsize(path)
            with open(path, "rb"):      # opens only if nothing else holds the file
                pass
        except OSError:
            size = -1                   # locked or vanished: not settled yet
        if size == last and size >= 0:
            return True
        last = size
        time.sleep(wait)
    return False

The size-stability check is the primary signal; the open attempt is a cheap complement. If a file is still copying, settled returns False and you simply leave it for the next pass - the sweep will pick it up once it is done.

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