eference,
          must have im_self and im_func attributes and be
          reconstructable via the following, which is true of built-in
          instance methods::

            target.im_func.__get__( target.im_self )

        - ``on_delete``: Optional callback which will be called when
          this weak reference ceases to be valid (i.e. either the
          object or the function is garbage collected).  Should take a
          single argument, which will be passed a pointer to this
          object.
        c