Am 13.01.2010 21:23, schrieb Allen Saunders:
>>
>> Calling COM methods is straightforward just like with other Python objects.
>> They can be called with positional and named arguments.
>>
>> Arguments marked [out] or [out, retval] in the IDL are returned from a
>> sucessful method call, in a tuple if there is more than one. If no [out]
>> or [out, retval] arguments are present, the HRESULT returned by the
>> method call is returned. When [out] or [out, retval] arguments are
>> returned from a sucessful call, the HRESULT value is lost.
>>
>> If the COM method call fails, a COMError exception is raised, containing
>> the HRESULT value.
>>
>
> Is it true that there is no way to get a successful HRESULT from a call that
> has [out] variables? An example is a function that can timeout, it may
> return either S_OK (0) or S_FALSE (1), an the difference between the two is
> important!
>
> Is there anyway for me to retrieve that 'lost' HRESULT?
Yes, there is indeed a way, but it is a little bit complicated.
For the following discussion lets assume a com interface with a method
like this (IDL code):
interface IInterface : IUnknown {
...
HRESULT DoSomething([in] int value, [out] int *result);
...
}
comtypes will create a class 'IInterface' with a method named 'DoSomething'.
This method has the signature 'result = DoSomething(value)', and returns the
[out]
parameter but not the HRESULT value (except when it fails).
Another private method that is created is named '__com_DoSomething' and has the
signature 'hresult = __com_DoSomething(value, presult)'. It takes two
arguments:
An integer and a POINTER(c_int) instance; and it will return the hresult value.
So, you can either call this method directly - but you must take the name
mangling
into account that Python does:
result = c_int()
hr = obj._IInterface__com_DoSomething(value, byref(result))
print hr
print result.value
Or you implement the DoSomething method yourself by adding this to the generated
code for the IInterface class (somewhere in comtypes\gen\<xxx>.py):
class IInterface(IUnknown):
...
def DoSomething(self, value):
result = c_int()
hr = self.__com_DoSomething(value, byref(result))
return hr, result.value
In this case name mangling is not needed (because the private method is called
from inside the class); and comtypes will not override your method
implementation.
This technique is used in some interface implementations in comtypes itself to
provide a more python interface to some methods when the automatic wrapping
doesn't work so good. Quite some examples are in comtypes\typeinfo.py,
for the ITypeLib and ITypeInfo interfaces.
--
Thanks,
Thomas
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