Many systems allow to monitor changes on a filesystem, but I could not find any that is both portable and compatible with gevent. For instance, watchdog is portable on Windows, Linux and MacOS, but it uses threads and does not work together with gevent. On the other hand, gevent_inotifyx, as its name says, works well with gevent, but it relies on inotify on so is for Linux only.

Here is a solution that is only based on gevent and should be as portable as gevent itself.

So far, this solution is buggy under Windows, an issue has been opened on gevent GitHub. The bug is that notifications arrive very slowly, I typically experience 30s delays between an update takes place and its notification is reported.
EDIT (July 3th 2015): there is few hope that is will ever work unless libev is updated on Windows, see the issue on GitHub for details

First, we import the required packages and work around a bug in Windows:

import os, os.path, collections
import gevent, gevent.pool, gevent.queue, gevent.hub

import platform
if platform.system() == "Windows" :
    # see https://github.com/gevent/gevent/issues/459
    import socket

Then we define a structure for events to provide an event name, a path on which the event occurred and a Boolean flag to know if the path is a directory or a file (this is needed for a delete event since there is no way to know what has been deleted):

event = collections.namedtuple("event", ["name", "path", "isdir"])

Class DirWatcher handles all the changes in a tree starting at root, it feeds a queue q with the observed events and provides a method get to retrieve the events:

class DirWatcher (object) :
    def __init__ (self, root) :
        self.root = os.path.abspath(root)
        self.workers = gevent.pool.Group()
        self.watched = set()
        self.q = gevent.queue.Queue()
        self.get = self.q.get
        self.add(self.root, "crawl")

Using os.path.abspath is important because libev clearly specifies that:

The path should be absolute: If it is relative and your working directory changes, then the behaviour is undefined.

The constructor also defined a group of workers greenlets, each of which will be in charge of one file or directory whose path is recorded in attribute watched (this will be useful latter on). Finally, the constructor calls method add to add the root directory to its watched objects, which works as follows:

    def add (self, path, evtname="create") :
        if os.path.isdir(path) :
            for name in os.listdir(path) :
                self.add(os.path.join(path, name), evtname)
        self.workers.spawn(self.watch, path)
        self.q.put(event(evtname, path, os.path.isdir(path)))

If the added object is a directory, it's children are added recursively. Then, a worker is spawned to watch the object and an event is generated to report the new object. Parameter evtname is the name of the event to trigger when an object is added to the DirWatcher. So, the add initiated from __init__ and the subsequent recursive crawling will generate events crawl. Latter, objects creation will generate events create.

Finally, the worker is as follows:

    def watch (self, path) :
        hub = gevent.get_hub()
        watcher = hub.loop.stat(path, 1)
        self.watched.add(path)
        isdir = os.path.isdir(path)
        if isdir :
            old = set(os.listdir(path))
        while path in self.watched :
            try :
                with gevent.Timeout(2) :
                    hub.wait(watcher)
            except gevent.hub.LoopExit :
                break
            except gevent.Timeout :
                continue
            if os.path.isdir(path) :
                new = set(os.listdir(path))
                for name in new - old :
                    self.add(os.path.join(path, name))
                old = new
            elif os.path.exists(path) :
                self.q.put(event("update", path, isdir))
            else :
                break
        if isdir :
            for name in old :
                self.watched.discard(os.path.join(path, name))
        self.watched.discard(path)
        self.q.put(event("delete", path, isdir))

The first two lines invoke the appropriate gevent machinery to create a watcher that will allow to be notified when stat changes on path. Parameter 1 in hub.loop.stat(path, 1) specifies a refresh rate for the platforms where updates are monitored using polling (Windows for example). Then, we record in isdir if path corresponds to a directory and if so, we record in old the current content of the directory.

Then the main loop starts. Basically, while the object exists, the loop waits for a change notification from the watcher. But, there exists changes that are not notified, typically if you delete directory foo that contains a file foo/bar, no notification is sent about foo/bar being deleted. We handle this from the worker that watches foo, after the while loop, by removing children objects from self.watched, which allows the corresponding workers to exit their while loop. But, if theses workers are blocked on hub.wait(watcher), it does not help, this is why we enclose this instruction inside a timeout so that when no notification comes within 2 seconds, we execute the continue and have chance to exit the while loop. (Note that the timeout is set to 2 to ensure that the watcher whose rate is 1 has a chance to detect a change.) Apart from the timeout, a possible exception is LoopExit that occurs when the root directory itself is deleted, so we catch it to exit the while loop.

When the watcher unblocks hub.wait, we proceed by checking what happened and there are three cases. (1) The object is a directory, we get its new content and we add each new object that appeared in the directory. Note that it may be useful to generate here an update event for the directory itself (removed objects are handler in case 3 by their own worker). (2) If the object exists and is not a directory, it is a file and we just generate an update event for it. (3) Otherwise, the object has been deleted, so we exit the while loop.

After the while loop, we have seen already that, for a directory, we must remove its children. Then, we remove the object itself from self.watched and generate the event for this deletion.