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Python for Windows (pywin32) Extensions
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pythoncom.Empty does not always generate VT_EMPTY #2149

Open Chronial opened 7 months ago

Chronial commented 7 months ago

Commit f60c64d528c84395ec1ce4bdd7be878006d864af 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: https://github.com/mhammond/pywin32/blob/f7d0a79a7ba3c83b8e6f6c62c617f4eb9770a1e9/com/win32com/src/oleargs.cpp#L1184-L1189

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.

mhammond commented 7 months ago

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.

Chronial commented 7 months ago

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.