[Python] Self-referencing class: concrete python class from C interface

I am trying to design a C interface which could easily be extended in Python (using ctypes). I've used the natural idiom in C:

struct format {
int (*can_open)(const char *filename);
struct format * (*open)(const char *filename);
void (*delete)(struct format *self);
int (*read)(struct format *self, char *buf, size_t len);
};


It works nicely if I want to extend this interface from C directly:

struct derived /* concrete implementation */
{
struct format base;
};


But what I would really like to do, is implement this interface from Python using ctypes. Here is what I have so far:

CANOPENFUNC = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_char_p)
#OPENFUNC = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_char_p)
#OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
#DELETEFUNC = ctypes.CFUNCTYPE(None, ctypes.c_void_p)
#READFUNC = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p)

def py_canopen_func( string ):
print "py_canopen_func", string
return 1

canopen_func = CANOPENFUNC(py_canopen_func)
#open_func = OPENFUNC( py_open_func)
#delete_func = DELETEFUNC(py_canopen_func)
#read_func = READFUNC(py_canopen_func)

class python_format(ctypes.Structure):
_fields_ = (
('can_open', CANOPENFUNC),
('open', OPENFUNC),
('delete', DELETEFUNC),
('read', READFUNC),
)
def __init__(self):
self.can_open = canopen_func
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)
def py_open_func2( string ):
print "py_open_func2", string
return ctypes.byref(self)
self.open = OPENFUNC( py_open_func2 )
#self.delete = delete_func
#self.read = read_func


Really I am struggling to define the prototype for OPENFUNC here. Technically it should be:

OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER(python_format), ctypes.c_char_p)


However I need to define python_format first, which in turns requires a definition for OPENFUNC.

Bonus point: what would be an actual function implementation ? For instance:

def func( str ): return None


or

def func( str ): i = python_format(); return ctypes.pointer(i)


both gives me:

class python_format(ctypes.Structure):
pass
OPENFUNC = ctypes.CFUNCTYPE(ctypes.POINTER( python_format ), ctypes.c_char_p)
OPENFUNC( func )
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: invalid result type for callback function


Is this related to this other issue ? If so should I change my initial C design, since I will not be able to return a pointer to a python_format instance from a callback ?

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