Why can't I create a COM object in a new thread in Python?

Question

I'm trying to create a COM Object from a dll in a new thread in Python - so I can run a message pump in that thread:

from comtypes.client import CreateObject
import threading

class MessageThread(threading.Thread):

    def __init__(self):
        threading.Thread.__init__(self)
        self.daemon = True

    def run(self):
        print "Thread starting"
        connection = CreateObject("IDMessaging.IDMMFileConnection")
        print "connection created"

a = CreateObject("IDMessaging.IDMMFileConnection")
print "aConnection created"
t = MessageThread()
t.start()

this is the error trace I get:

aConnection created
Thread starting
>>> Exception in thread Thread-1:
Traceback (most recent call last):
  File "c:\python26\lib\threading.py", line 532, in __bootstrap_inner
    self.run()
  File "fred.py", line 99, in run
    self.connection = CreateObject("IDMessaging.IDMMFileConnection")
  File "c:\python26\lib\site-packages\comtypes\client\__init__.py", line 235, in CreateObject
    obj = comtypes.CoCreateInstance(clsid, clsctx=clsctx, interface=interface)
  File "c:\python26\lib\site-packages\comtypes\__init__.py", line 1145, in CoCreateInstance
    _ole32.CoCreateInstance(byref(clsid), punkouter, clsctx, byref(iid), byref(p))
  File "_ctypes/callproc.c", line 925, in GetResult
WindowsError: [Error -2147221008] CoInitialize has not been called

Any ideas?

Answer 1

You need to have called CoInitialize() (or CoInitializeEx()) on a thread before you can create COM objects on that thread.

from win32com.client.pythoncom import CoInitialize
CoInitialize()

Answer 2

As far as I remember (long time ago I'e programmed a lot with COM Components) you have to call CoInitialize on each thread if your COM Object uses STA.

http://msdn.microsoft.com/en-us/library/ms678543(VS.85).aspx

But I've no idea how to call that function in python.

Here is the MSDN Doc

http://msdn.microsoft.com/en-us/library/ms678543(VS.85).aspx