How to return a IStream COM object?

[Juan Carlos CORUÑA]

Hello all,

I have developed an automation server using Mark's win32 extensions
and I must return a IStream COM object from one method of the
automation server.

Here is an extract of my code:

class Stream:
	_public_methods_ = [ 'Read', 'Write', 'read', 'write',
						 'Seek', 'SetSize', 'CopyTo',
						 'Commit', 'Revert', 'LockRegion', 'UnlockRegion',
						 'Stat', 'Clone']
	_com_interfaces_ = [ pythoncom.IID_IStream ]

	def __init__(self, mode, data):
		print "init stream"
		self._data = data
		self._ptr = 0
		self._size = len(data)
		self._creationTime = Time(time())
		self._accessTime = Time(time())
		self._modificationTime = Time(time())
		self._mode = mode

	def Read(self, numBytes):
		"""Read the specified number of bytes from the string"""
		self._accessTime = Time(time())
		data = self._data[self._ptr:self._ptr + numBytes]
		self._ptr = self._ptr + numBytes
		if self._ptr > len(self._data): self._ptr = len(self._data) - 1
		print "Read data", data
		return data # string
	read = Read

	def Write(self, data):
		"""Write data to stream"""
		self._modificationTime = Time(time())
		print "Write data", data
		self._data = self._data + data
		return
	write = Write

	def Seek(self, offset, origin):
		"""Changes the seek pointer to a new location"""
		self._ptr = offset + origin
		return self._ptr # ULARGE_INTEGER

	def SetSize(self, newSize):
		"""Changes the size of the stream object."""
		self._size = newSize
		return

	def CopyTo(self, stream, cb):
		"""Copies a specified number of bytes from the
		current seek pointer in the stream to the current
		seek pointer in another stream"""
		data = self._data[self._ptr:self._ptr+cb]
		stream.Write(data)
		return len(data) # ULARGE_INTEGER

	def Commit(self, flags):
		"""Ensures that any changes made to a stream object
		open in transacted mode are reflected in the parent storage"""
		return

	def Revert(self):
		"""Discards all changes that have been made to a
		transacted stream since the last PyIStream::Commit call"""
		return

	def LockRegion(self, offset, cb, lockType):
		"""Restricts access to a specified range of bytes
		in the stream"""
		return

	def UnlockRegion(self, offset, cb, lockType):
		"""Removes the access restriction on a range of bytes
		previously restricted with PyIStream::LockRegion"""
		return

	def Clone(self):
		"""Creates a new stream object with its own seek pointer
		that references the same bytes as the original stream"""
		# not yet implemented
                return # PyIStream

	def Stat(self, grfStatFlag):
		"""Returns information about the stream"""
		st = ('stream', STGTY_STREAM, self._size, self._modificationTime,
			  self._creationTime, self._accessTime, self._mode, LOCK_EXCLUSIVE,
			 )
		return st # STATSTG

class AutomationServer:
	def GetStream(self):
		from win32com import storagecon
		_grfMode_READ = storagecon.STGM_SHARE_EXCLUSIVE | \
			storagecon.STGM_DIRECT | \
			storagecon.STGM_READ
		_grfMode_WRITE = storagecon.STGM_SHARE_EXCLUSIVE | \
			storagecon.STGM_WRITE
		s = XMLRPCLib2.Stream(_grfMode_WRITE, '')
		return wrap(s, pythoncom.IID_IStream, useDispatcher=useDispatcher)

I'm trying to use this automation server in the Navision ERP. Navision
accepts the return value VT_UNKNOWN and maps it to an Navision
internal type InStream or OutStream. These types must be standard
IStream COM objects.
Since I'm using a correct typelib calling GetStream does not generate
a exception, the problem appears when I call a method of the Navision
type InStream/OutStream accessing the Stream methods. The error
message says something like "The is an problem communicating with the
stream". The python trace collector says nothing.

Since there are working automation servers that returns a IStream COM
object for Navision, I think there must be an error in my Stream
class. I suppose that python maps the native IStream methods to the
PyIStream methods in order to avoid using the pointers of the native
interface.

I made some test using pythoncom._univgw in order to create a vtable
for the IStream COM object, but the function CreateTearOff crashes
python. So, running testuniv.py after changing the line "import
univgw" to "from pythoncom import _univgw as univgw" crashes python. A
bug? Can anybody confirm this?

Can anybody help me? Maybe a working example?