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The ctypes documentation describes the '._as_parameter_' attribute, which simplifies using Python objects to encapsulate C objects. If a ctypes object with the '._as_parameter_' attribute defined is passed as a parameter to a C function, the value of '._as_parameter_' is supplied to the C function instead of a pointer to the Python project.
At least that's how it's advertised to work. And it does indeed work as advertised on Linux, but on Mac OS X the mechanism doesn't appear to work correctly. I have created a small dummy example demonstrating this at https://github.com/standage/ctypes-demo.

FWIW, your test does seem to run if the C test program is instead built in 32-bit mode (modifying CFLAGS to include "-arch i386"). For example, with the Apple OS X 10.10 system Python:
DYLD_LIBRARY_PATH=.. arch -i386 /usr/bin/python2.7 test.py
Gizmo 1: 15
Gizmo 2: 20
Empty data

c_void_p's getfunc returns the address as an integer. In turn, an integer argument gets converted to a C int. This mean you need to a create a new c_void_p from the address for _as_parameter_, e.g.
 self._as_parameter_ = c_void_p(self.g)
Or set restype to a subclass of c_void_p. ctypes won't call the getfunc in this case, e.g. 
 class my_void_p(c_void_p):
 pass
 mylib.my_gizmo_create.restype = my_void_p
ConvParam
https://hg.python.org/cpython/file/ee879c0ffa11/Modules/_ctypes/callproc.c#l613
GetResult
https://hg.python.org/cpython/file/ee879c0ffa11/Modules/_ctypes/callproc.c#l903 

Thanks, eryksun. Any objections to closing this issue?

Sorry for my radio silence. Your example was very helpful, and restores the behavior I was looking for. Thanks!