I may be saved. I'm still not sure exactly why, though.
After some desperate tracing, I found something that works ( I found an 'if'
statement when calling the method which always went down one path when I
passed in an LP_LP_c_double instance. I forced it down the other path by
passing in an instance of LP_SAFEARRAY_c_double, as shown below):
>>> import ctypes
>>> import comtypes.safearray as safearray
>>> safearray
<module 'comtypes.safearray' from
'C:\Python25\lib\site-packages\comtypes\safearray.pyc'>
>>> itrace = bench.SA.Traces[u'Trace1']
>>> itrace
<POINTER(IIviSpecAnTrace) ptr=0x2216610 at 107a8a0>
>>> AmplitudeType = safearray._midlSAFEARRAY(ctypes.c_double)
>>> AmplitudeType
<class 'ctypes.LP_SAFEARRAY_c_double'>
>>> amplitude = AmplitudeType()
>>> amplitude
<ctypes.LP_SAFEARRAY_c_double object at 0x00FB3DF0>
>>> itrace.FetchY(amplitude)
<ctypes.LP_SAFEARRAY_c_double object at 0x00FB3DF0>
and amplitude.unpack() gives me what I was looking for all along. I've
actually found several different routes to the same end, as well, using the
underlying SafeArray* functions in oleaut32.dll and the ctypes structures
tagSAFEARRAYBOUNDS and tagSAFEARRAY and fun with cast, from_address, and
address of. I actually learned a lot doing this.
After much digging into pointers and arrays in ctypes, one question remains
for me (and I have just checked out the ctypes trunk to try to figure this
out myself) is what the mechanics of classes defined as POINTER(
<COM_interface_class> ) work. That is, there's no contents attribute
(something I looked at briefly, again in desperation, to get access to the
wrapped COM interface), they can't be indexed, so on and so forth. I figure
this must be an interaction of the _Pointer class, from which types
constructed with POINTER inherit, and the metaclass of COM interface
classes. Any commentary is welcome.
Man, I love this module.
Matt
On Sat, May 10, 2008 at 3:05 PM, <
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> Today's Topics:
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> 1. SAFEARRAY SNAFU (Matt Wilbur)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 9 May 2008 16:26:42 -0400
> From: "Matt Wilbur" <[EMAIL PROTECTED]>
> Subject: [comtypes-users] SAFEARRAY SNAFU
> To: comtypes-users@lists.sourceforge.net
> Message-ID:
> <[EMAIL PROTECTED]>
> Content-Type: text/plain; charset="iso-8859-1"
>
> Hello All, especially Thomas,
>
> I'm running into a fairly serious snag calling a method that is supposed
> to
> take a SAFEARRAY** as input. The class generated from GetModule is
>
> IIviSpecAnTrace._methods_ = [
> COMMETHOD([helpstring(u'This function returns the trace the spectrum
> analyzer aquires.')], HRESULT, 'FetchY',
> ( ['in', 'out'], POINTER(_midlSAFEARRAY(c_double)),
> 'Amplitude' )),
>
> <other methods omitted>
>
> ]
>
>
> >From the header file its:
>
> public:
> virtual /* [helpcontext][helpstring] */ HRESULT STDMETHODCALLTYPE
> FetchY(
> /* [out][in] */ SAFEARRAY __RPC_FAR * __RPC_FAR *Amplitude) =
> 0;
>
>
>
> Everything was going swimmingly until I tried to use this function.
> Here's
> what I do:
>
> In [305]: safe2 = (c_double*401)()
>
> In [306]: safe2
> Out[306]: <__main__.c_double_Array_401 object at 0x0292A670>
>
> I tried manually creating the SAFEARRAY and passing it to the FetchY
> method,
> but I got bounced because it looked like the underlying Python code was
> constructing it for me.
>
> However, the array, when returned, is filled with 0.0. In fact, whatever
> I
> fill the array with, it remains unchanged after the call.
>
> I've verified things work correctly at the C++ level, and I've verified
> the
> instrument is communicating and sending back the right values. I'm
> obviously buggering something up, but after a day and a half of
> investigation, I have to admit I'm stumped. I don't know enough about the
> internals of comtypes to debug this, so I'm hoping Thomas can help me out.
>
> Matt
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