Mine

cdef extern from "Python.h":
    int PyObject_AsReadBuffer(obj, void **buffer, Py_ssize_t *buffer_len) except -1

cdef class Packet:

    def __init__(self, dataContents = "", flags = 0):
        cdef char *buf
        cdef Py_ssize_t len
        PyObject_AsReadBuffer(dataContents, <void **>&buf, &len)

        # Safe - enet_packet_create copies the buffer
        self._enet_packet = enet_packet_create(buf, len, flags)