python, COM and multithreading issue

Question

I'm trying to take a look at IE's DOM from a separate thread that dispatched IE, and for some properties I'm getting a "no such interface supported" error. I managed to reduce the problem to this script:

import threading, time

import pythoncom
from win32com.client import Dispatch, gencache
gencache.EnsureModule('{3050F1C5-98B5-11CF-BB82-00AA00BDCE0B}', 0, 4, 0) # MSHTML

def main():
    pythoncom.CoInitializeEx(0)
    ie = Dispatch('InternetExplorer.Application')
    ie.Visible = True
    ie.Navigate('http://www.Rhodia-ecommerce.com/')
    while ie.Busy:
        time.sleep(1)

    def printframes():
        pythoncom.CoInitializeEx(0)
        document = ie.Document
        frames = document.getElementsByTagName(u'frame')
        for frame in frames:
            obj = frame.contentWindow

    thr = threading.Thread(target=printframes)
    thr.start()
    thr.join()

if __name__ == '__main__':
    thr = threading.Thread(target=main)
    thr.start()
    thr.join()

Everything is fine until the frame.contentWindow. Then bam:

Exception in thread Thread-2:
Traceback (most recent call last):
  File "C:\python22\lib\threading.py", line 414, in __bootstrap
    self.run()
  File "C:\python22\lib\threading.py", line 402, in run
    apply(self.__target, self.__args, self.__kwargs)
  File "testie.py", line 42, in printframes
    obj = frame.contentWindow
  File "C:\python22\lib\site-packages\win32com\client\__init__.py", line 455, in __getattr__
    return self._ApplyTypes_(*args)
  File "C:\python22\lib\site-packages\win32com\client\__init__.py", line 446, in _ApplyTypes_
    return self._get_good_object_(
com_error: (-2147467262, 'No such interface supported', None, None)

Any hint ?

Orthogonal to your question: update to Python2.7; try selenium — jfs
– jfs, Commented Jan 3, 2012 at 18:47
Not an option right now; I'm in a commercial context and we do much more than what Selenium does. We're on the way to upgrade Python though :) We found a workaround by using apartment instead of free threading and marshalling stuff by hand but I'm still curious. — fraca7
– fraca7, Commented Jan 4, 2012 at 7:59

Ben · Accepted Answer · 2012-01-04 11:33:23Z

The correct answer is to marshal stuff by hand. That's not a workaround it is what you are supposed to do here. You shouldn't have to use apartment threading though.

You initialised as multithreaded apartment - that tells COM that it can call your interfaces on any thread. It does not allow you to call other interfaces on any thread, or excuse you from marshalling interfaces provided by COM. That will only work "by accident" - E.g. if the object you are calling happens to be an in-process MTA object, it won't matter.

CoMarshalInterThreadInterfaceInStream/CoGetInterfaceAndReleaseStream does the business.

The reason for this is that objects can provide their own proxies, which may or may not be free-threaded. (Or indeed provide custom marshalling). You have to marshal them to tell them they are moving between threads. If the proxy is free threaded, you may get the same pointer back.

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