import gc
import ctypes
import objgraph
class libusb_transfer(ctypes.Structure):
    pass

libusb_transfer_p = ctypes.POINTER(libusb_transfer)
libusb_transfer_cb_fn_p = ctypes.CFUNCTYPE(None, libusb_transfer_p)
libusb_transfer._fields_ = [('callback', libusb_transfer_cb_fn_p)]
class USBTransfer(object):
    __transfer = None
    __ctypesCallbackWrapper = None
    def __init__(self):
        self.__transfer = libusb_transfer()
        self.__ctypesCallbackWrapper = libusb_transfer_cb_fn_p(
            self.__callbackWrapper)
    def close(self):
        self.__ctypesCallbackWrapper = None
        # break the cycle (but brutal)
        #self.__transfer = None
        # does not break the cycle (bug ?)
        #self.__transfer.callback = None
        #self.__transfer.callback = libusb_transfer_cb_fn_p(None)
        # breaks the cycle (but overcomplex)
        #self.__transfer.callback = libusb_transfer_cb_fn_p(lambda x: None)
    def setBulk(self):
        self.__transfer.callback = self.__ctypesCallbackWrapper
    def __callbackWrapper(self, transfer_p):
        pass

t = USBTransfer()
i = id(t)
t.setBulk()
t.close()
t = None
objgraph.show_backrefs([objgraph.at(i)], 20)