pythoncom.Empty does not always generate VT_EMPTY

[Christian Fersch]

Commit f60c64d tried to fix the behavior pythoncom.Empty, so using it generates VT_EMPTY variants instead of VT_ERROR:DISP_E_PARAMNOTFOUND, for which a new pythoncom.ArgNotFound constant was introduced.

But the change missed one case:

pywin32/com/win32com/src/oleargs.cpp

Lines 1184 to 1189 in f7d0a79

 if (obj->ob_type == &PyOleEmptyType) { 
     // Quick exit - use default parameter 
     V_VT(var) = VT_ERROR; 
     V_ERROR(var) = DISP_E_PARAMNOTFOUND; 
     return TRUE; 
 } 

So, depending on where you use it, pythoncom.Empty is sometimes converted to VT_EMPTY and sometimes to VT_ERROR.

One example of where this happens is calling IDispatch interfaces via pywin32.

As a workaround, you can manually construct a VT_EMPTY: win32com.client.VARIANT(pythoncom.VT_EMPTY, 0)

Note that there is some code in pywin32 that relies on the current behavior, so when fixing this issue, some uses of pythoncom.Empty need to be replaced by pythoncom.ArgNotFound.

[Mark Hammond]

Unfortunately way back in the day there were some objects which relied on the current behaviour, which might make this tricky to change without breaking code. I don't have office locally, but I'd be a lot more confident if we could assure ourselves the most of the test scripts (eg, testOffice.py) worked with current versions of those apps.

[Christian Fersch]

I thought so – that's why I didn't create a PR for this. I also didn't "encounter" this issue – I just noticed it while debugging for #2150. Unless somebody actually encounters this in the wild, it's probably simpler to just leave it as is for now.

I mostly created this Issue to document what I noticed.