在Python中使用Watchdog处理图像

Question

我正在使用watchdog这个库来监测一个特定文件夹里新图片的生成。当watchdog发现有新图片创建时，我就会启动一些图像处理的功能，使用SimpleCV或OpenCV。

不过，这些图片是通过树莓派相机拍摄的。从下面的错误信息来看，我觉得在图片刚出现在文件夹时，整个图片并没有完全保存下来。（简单来说，文件是分成“几块”保存的，或者说是多种格式的。）

请注意，当我复制和粘贴一张图片到相应的文件夹时，脚本能够顺利运行。

问题：有没有办法确保只有在文件完全保存后再开始图像处理？

import time
from SimpleCV import Image
from watchdog.observers import Observer
from watchdog.events import FileSystemEventHandler

class ExampleHandler(FileSystemEventHandler):
    def on_created(self, event):
        image = Image(event.src_path)
        do image processing stuff
        print "Got event for file %s" % event.src_path 

observer = Observer()
event_handler = ExampleHandler() # create event handler
observer.schedule(event_handler, path='/path/to/images') # set observer to use created handler in directory
observer.start()

# sleep until keyboard interrupt, then stop + rejoin the observer
try:
    while True:
        time.sleep(1)
except KeyboardInterrupt:
    observer.stop()

observer.join()

错误信息：

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/local/lib/python2.7/dist-packages/watchdog/observers/api.py", line 241, in run
    self.dispatch_events(self.event_queue, self.timeout)
  File "/usr/local/lib/python2.7/dist-packages/watchdog/observers/api.py", line 408, in dispatch_events
    self._dispatch_event(event, watch)
  File "/usr/local/lib/python2.7/dist-packages/watchdog/observers/api.py", line 403, in _dispatch_event
    handler.dispatch(event)
  File "/usr/local/lib/python2.7/dist-packages/watchdog/events.py", line 361, in dispatch
    _method_map[event_type](event)
  File "picture_classifier_cube.py", line 11, in on_created
    image = Image(event.src_path)
  File "/usr/local/lib/python2.7/dist-packages/SimpleCV/ImageClass.py", line 1073, in __init__
    self._pil = pil.open(self.filename).convert("RGB")
  File "/usr/lib/python2.7/dist-packages/PIL/Image.py", line 1980, in open
    raise IOError("cannot identify image file")
IOError: cannot identify image file

编辑 在更新处理程序后，让它在识别到新文件后暂停几秒钟，我收到了不同的错误。

class ExampleHandler(FileSystemEventHandler):
    def on_created(self, event):
        time.sleep(3)
        cans = Image(event.src_path)

错误

IOError: [Errno 2] No such file or directory: '/path/to/images/test.jpg~'

请注意，我是用以下命令来捕捉图片的：raspistill -o 'test.jpg'

Answer 1

虽然没有办法绝对确定一个文件是否已经写完，而不去修改写入这个文件的程序，但你可以通过监控文件的大小来判断：

from os import stat
from time import sleep

def wait_for_write_finish( filename ):
    last_size, size= -1, 0
    while size!=last_size:
        sleep(1)
        last_size, size= size, stat(filename).st_size

你可能需要使用合适的线程来实现这个功能。