Hi,
Recently we noticed the same problem with our project.
We access the MS Word object model via IDispatch, but there is a
GetPoint method which takes left, top, hight and width as byref arguments.
As comtypes doesn't support vt_byref out of the box, our first idea was
to rewrite the code for VARIANT._set_value(), and provide comtypes with
a patch.
However, looking at VARIANT._set_value():
There are lots of returns all the way through the function, so its
clear it would have to be rewritten almost from the ground up. And since
I'm not too skilled with all of the VARIANT types, I don't want to break
any existing logic.
So, what we currently do, is when our project imports comtypes at
initialization time, we do the following:
#Monkey patch comtypes to support byref in variants
from comtypes.automation import VARIANT, VT_BYREF
from ctypes import cast, c_void_p
from _ctypes import _Pointer
oldVARIANT_value_fset=VARIANT.value.fset
def newVARIANT_value_fset(self,value):
realValue=value
if isinstance(value,_Pointer):
try:
value=value.contents
except (NameError,AttributeError):
pass
oldVARIANT_value_fset(self,value)
if realValue is not value:
self.vt|=VT_BYREF
self._.c_void_p=cast(realValue,c_void_p)
VARIANT.value=property(VARIANT.value.fget,newVARIANT_value_fset,VARIANT.value.fdel)
So, from then on, when ever passing an instance of a POINTER of a ctypes
type, comtypes will automatically treet the value as byref.
Some example code for GetPoint:
from ctypes import c_long, pointer
rangeLeft=c_long()
rangeTop=c_long()
rangeWidth=c_long()
rangeHeight=c_long()
self.obj.WinwordWindowObject.getPoint(pointer(rangeLeft),pointer(rangeTop),pointer(rangeWidth),pointer(rangeHeight),self._rangeObj)
I would probably prefer that we use ctypes.byref, rather than
ctypes.POINTER, as the name relates better to what its doing. But,
ctypes.byref(x) yields an object who's type is something like
type(cArgObject), which I can't find anywhere in ctypes or _ctypes. So,
it would be a little hard for VARIANT._set_value to properly identify
that value was a byref.
Interestingly enough though, if comtypes compiles the actual COM
interfaces for the MS Word object model, POINTER is used anyway, so I
guess POINTER is ok.
Mick
On 13/02/2010 10:48 AM, Pablo Bianucci wrote:
> Hi Thomas!
>
> On Fri, 12 Feb 2010, Thomas Heller wrote:
>
>> Exactly. I cannot provide a patch, but here's a code snippet that creates
>> such a beast:
>
> Yes! It works!!
>
> This is the final working snippet:
>
> <snip>
> travel = c_float(0)
> v = VARIANT()
> v._.c_void_p = cast(pointer(travel), c_void_p)
> v.vt = VT_BYREF | VT_R4
>
> mode = c_int(0)
> vm = VARIANT()
> vm._.c_void_p = cast(pointer(mode), c_void_p)
> vm.vt = VT_BYREF | VT_I4
>
> try:
> piezo1.GetMaxTravel(APTPiezoLib.CHAN1_ID, v)
> piezo1.GetControlMode(APTPiezoLib.CHAN1_ID, vm)
> except Exception, e:
> print "Exception:", e
> print travel.value, mode.value
> </snip>
>
> If you are not careful when setting the types you get a "Type Mismatch"
> error (even though the protoypes in the help were the same, I had to use
> different types for the two funcions there).
>
> I wonder why you can't integrate this into the package. I'm fine as this,
> as this will let me write the program I need, but it would be more user
> friendly to be able use pointers as arguments for the method invocation.
> But I don't understand COM internals, so it could be that implementing
> that could lead to other problems.
>
> Thank you very much for your help!
>
> Bye& Good Luck!
>
> Pablo.
>
> P.S.: If you visit Canada and stay at a city I happen to be living in,
> consider yourself invited to a drink. ;-)
>
>
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--
Michael Curran
President, NV Access Inc
Email: m...@nvaccess.org
Website: www.nvaccess.org
ABN 61773362390.
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