[python-win32] Calling an OLE Automation (IDispatch) server which uses "out params"

[Mark Hammond]

On 13/12/2012 11:36 AM, Dave Calkins wrote:
> I'd like to be able to call an OLE Automation (IDispatch) server from
> python to automate an existing application.  I'm not at liberty to
> change the existing automation interface provided by the application so
> I need to get it to work with what the app currently exposes.
>
> The interface I'm attempting to work with makes use of "out params".
> Here's an excerpt from the IDL for a small sample app I created to
> reproduce the issue.
>
> ==========
>      dispinterface IMyApp
>      {
>          properties:
>
>          methods:
>              [id(1)] void ShowMessage(BSTR msg);
>              [id(2)] void SetSettingValue(BSTR settingName, BSTR
> settingValue);
>              [id(3)] void GetSettingValue(BSTR settingName, BSTR*
> settingValue);
>      };
> ==========
>
> The first two methods are straightforward and I had no problem calling
> them from python.  The problem is the third one, GetSettingValue, since
> it requires you to pass in a reference to a variable which it then fills
> in for you.
>
> I read through CH12 of "Python Programming on Win32" by Hammond/Robinson
> as a guide.  Based on that I installed pywin32 and then tried running
> MakePy.  Here's an excerpt of the code generated by MakePy.
>
> ==========
>      def GetSettingValue(self, settingName=defaultNamedNotOptArg,
> settingValue=defaultNamedNotOptArg):
>          return self._oleobj_.InvokeTypes(3, LCID, 1, (24, 0), ((8, 0),
> (16392, 0)),settingName
>              , settingValue)
>
>      def SetSettingValue(self, settingName=defaultNamedNotOptArg,
> settingValue=defaultNamedNotOptArg):
>          return self._oleobj_.InvokeTypes(2, LCID, 1, (24, 0), ((8, 0),
> (8, 0)),settingName
>              , settingValue)
>
>      def ShowMessage(self, msg=defaultNamedNotOptArg):
>          return self._oleobj_.InvokeTypes(1, LCID, 1, (24, 0), ((8,
> 0),),msg
>              )
> ==========
>
> Comparing what it generated for GetSettingValue vs. SetSettingValue I
> noted the difference of 16392 vs. 8.  Looking up those in the windows
> headers it appears those numeric constants correspond to
> VT_BYREF|VT_BSTR vs. VT_BSTR.  So MakePy is generating something
> different for the "out param" and it seems to even be noticing that its
> by reference.
>
> I haven't figured out how to actually call this, though.  In CH12 of the
> book I mentioned, it is noted that for such methods the params are
> converted to return values instead.  The example given in the book is:
>
> ==========
> ok = GetSize( &left, &right, &top, &bottom);
>       becomes
> left, right, top, bottom = GetSize()
> ==========
>
> It doesn't appear that this is happening with the MakePy results I'm
> getting.  But clearly MakePy is detecting the "by reference" parameter.
>
> Here's the output I get when attempting to call this method.  I just
> sort of guessed at how I might call that method.
>
> ==========
> Python 2.7.3 (default, Apr 10 2012, 23:31:26) [MSC v.1500 32 bit
> (Intel)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
>  >>> import win32com.client
>  >>> x = win32com.client.Dispatch("MyApp.Application")
>  >>> x.ShowMessage("Hello, world")
>  >>> x.SetSettingValue("sName","sValue")
>  >>> sValue = ""
>  >>> x.GetSettingValue("sName",sValue)
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<COMObject MyApp.Application>", line 2, in GetSettingValue
> pywintypes.com_error: (-2147352571, 'Type mismatch.', None, 2)
>  >>> sValue = x.GetSettingName("sName")

This is the form you should use, but the method name you are trying to 
call is "GetSettingValue", not "GetSettingName", hence the AttributeError.

Mark

> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File
> "C:\Users\dave\apps\Python273\lib\site-packages\win32com\client\dynamic.py",
> line 522, in __getattr__
>      raise AttributeError("%s.%s" % (self._username_, attr))
> AttributeError: MyApp.Application.GetSettingName
>  >>>
> ==========
>
> Any advice on how I can make use of this method, which takes "out
> params", from python?
>
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