How to use the deltachat.capi.ffi.def_extern function in deltachat

@capi.ffi.def_extern()
def py_dc_callback(ctx, evt, data1, data2):
    """The global event handler.

    CFFI only allows us to set one global event handler, so this one
    looks up the correct event handler for the given context.
    """
    try:
        callback = _DC_CALLBACK_MAP.get(ctx, lambda *a: 0)
    except AttributeError:
        # we are in a deep in GC-free/interpreter shutdown land
        # nothing much better to do here than:
        return 0

    # the following code relates to the deltachat/_build.py's helper
    # function which provides us signature info of an event call
    evt_name = get_dc_event_name(evt)