'Run the callback unless it has already been called or cancelled'

def __call__(self, wr=None):

    try:
        del _finalizer_registry[self._key]
    except KeyError:
        sub_debug('finalizer    no    longer    registered')
    else:
        sub_debug('finalizer    calling    %s    with    args    %s    and    kwargs    %s', self._callback, self._args, self._kwargs)
        res = self._callback(*self._args, **self._kwargs)
        self._weakref = self._callback = self._args = self._kwargs = self._key = None
        return res
    return
