[python-win32] Input-only vs. In/Out Function Parameters

[Smith, Brian (CR)]

Yup, still trying to figure this one out but I've got a little bit more 
information.

To summarise:  I created a Python COM Server to be used by Visual Basic
and found an issue with in/out parameters because of the lack of type
information.  To fix this, I created a type library but this led to the
following two problems:
 
1) The function calls on the COM object failed unless I explicitly returned
   an HRESULT in addition to returning the other output parameters.
2) After adding an HRESULT as the first element of the returned tuple, I
   found that the *last* element of the returned tuple was being used as
   the function's return value instead of the *first* value (or second after
   the HRESULT?) as expected.
 
I've now tried registering the COM object with --debug and viewed the trace
output from PythonWin's trace collector utility.  After calling a function
that does not add an HRESULT, I ended up getting the following traceback:
 
Traceback (most recent call last):
  File "C:\DevTools\Python23\lib\site-packages\win32com\universal.py", line
188, in dispatch
      raise TypeError, "Expected %s return values, got: %s" %
(len(meth._gw_out_args) + 1, len(retVal))
      exceptions.TypeError: Expected 3 return values, got: 2
 
Looking at universal.py, I found this in the dispatch() function of
class Definition:
 
    ...
    if type(retVal) == types.TupleType: # Like pythoncom, we special case a
tuple.
        # However, if they want to return a specific HRESULT,
        # then they have to return all of the out arguments
        # AND the HRESULT.
        if len(retVal) == len(meth._gw_out_args) + 1:
            hr = retVal[0]
            retVal = retVal[1:]
        else:
            raise TypeError, "Expected %s return values, got: %s" %
(len(meth._gw_out_args) + 1, len(retVal))
    else:
        retVal = [retVal]
        retVal.extend([None] * (len(meth._gw_out_args)-1))
        retVal = tuple(retVal)
    WriteFromOutTuple(retVal, meth._gw_out_args, argPtr)
    return hr
 
I'm not sure what the original intent was, but the wording in the comment 
seems to imply that returning a specific HRESULT should be optional. 
However, the code seems to make it mandatory and this would explain
problem #1.  I'm not sure if this is a bug or by design.
 
I also tried looking at the source for WriteFromOutTuple() to determine the 
cause of problem #2, but with limited SWIG knowledge I didn't get far. It 
seems like the function verifies data types, but I didn't see where any 
"Writing" actually occurred.
 

Brian
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