python-cffi-example | simple example Python CFFI package

See https://cffi.readthedocs.io/en/latest/using.html#extern-python-and-void-arguments. Avoid using PyObject * with CFFI. The correct answer is to keep int cmp_function(const void*, const void*);. I will assume that you already use ffi.new_handle() to convert the objects to the void * equivalent that you can send to C (in the add_to_hash_table() method, etc.) and ffi.from_handle() to read them back (in the get_from_hash_table() method, etc.).

To implement the comparison function, you declare a function in Python with extern "Python", as in the above document. Then you use that function for the cmp_function pointer in your struct. Inside that Python function, you receive the two arguments as two void *. You first convert them back to the original Python objects with ffi.from_handle(), and then just use regular Python---in your case, probably just if x == y: return 1; else: return 0 or similar.

Be careful about keeping the Python objects alive after calling ffi.new_handle(). They are not kept alive just by the fact that their void * representation happens to be stored inside some C structures.

Note that CFFI is not quite well suited to implement pure data structures, because all these conversions and multiple storages that increase the overhead when compared with the direct CPython-C-API approach.