[python-win32] passing array by reference (3)

[Peter Enenkel]

Hello Tim,

I concur that the array SHOULD be a return value (at the very least in
Python). Unfortunately it isn't. Definitely not with late binding and I
think not with early binding. The function does expect a parameter of the
described type and does not return anything.

In my opinion it would theoretically be possible to process an argument by
reference in Python, of cause it would have to be a mutable object. Which,
as you mentioned, tuples are not, so it would have to be restricted to
lists or some specialized transport object. Admittedly, any such solution
would not be particularly pythonic. I thought that the pythoncom.VARIANT
class is such a transport object. But I could not get it to work.

I suspect that you are correct that the interface might not be declared
properly on the serverside. On the other hand similar problems with passing
arrays by reference seem to have cropped up with other COM servers also.

I have access to the official Catia V5 automation IDL documentation (which
is reasonably complete) as well as the .tlb files. But I do not know what I
could do with the later.

Unfortunately Catia has deemed it necessary to split the COM server into
several (more that two dozen) files which doesn't exactly help with early
binding and identifying the required interfaces. That is probably one of
the reasons I could not get it to work with early binding. An other
difficulty with that is, that (interface) inheritance is used, i.e. both
PartDocument and ProductDocument both inherit from Document but each has
additional attributes. Functions always seem to return the base type which
then would require some kind of cast.

I have managed to implement a workaround in the manner that Andreas Holtz
suggested. That is, using win32com to instruct Catia to execute a VBScript
function which calls GetComponents with an ByRef argument and then returns
it as a return value, which in turn is passed back to python. Not a
particularly elegant solution, but it works! So the pressure to find a
"native" solution isn't quite as high as before. Nevertheless I'm still
interested in a way to do it "right". If you have any ideas in that
direction I'd appreciate hearing them.

best regards
Peter




2017-02-20 19:07 GMT+01:00 Tim Roberts <timr at probo.com>:

> Peter Enenkel wrote:
> >
> > I'm trying to read some data from CATIA v5 (which is an older CAD
> > application) via its COM server. Overall it works rather well, even if
> > I'm fairly new to COM communication.
> >
> > Nevertheless I have run into a problem: One specific function
> > *GetComponents * is supposed to return its results via an array passed
> > by reference. As far as I can tell it is exactly the same problem as
> > in
> > (1)https://mail.python.org/pipermail/python-win32/2002-
> November/000562.html
> > and
> > (2)https://mail.python.org/pipermail/python-win32/2014-July/013234.html
> > unfortunately there doesn't appear to be a solution as of yet.
> >
> > The python call looks something like this
> > a = list(range(0,12))
> > b = catapp.ActiveProduct.Position.GetComponents(a)
> > print(a) # outputs (0,1,2,3,4,5,6,7,8,9,10,11) instead of the actual
> > values
>
> Usually, in this kind of situation, the array would be an output parameter:
>
>     b, a = catapp.ActiveProduct.Position.GetComponents()
>
> Remember that Python doesn't actually do pass-by-reference in the way
> you are used to.  In your code above, you aren't really passing "a".
> What you're passing is the list that "a" is bound to.  The Python COM
> code converts that into whatever type the function wants.  If that array
> gets changed, it doesn't change the list.  It can't, because that
> parameter might actually have been a tuple, which can't be modified, and
> there is no way for the function to change the binding of "a".  That's
> why the COM stuff usually returns the changed parameter, so you can
> change your binding.
>
> Do you have the actual function signature in C++ or IDL form?  If they
> haven't declared it properly, PythonCOM might not be able to determine
> that this is an output parameter.
>
> --
> Tim Roberts, timr at probo.com
> Providenza & Boekelheide, Inc.
>
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