'`NDData` : reference to `NDData` instance with this uncertainty.
In case the reference is not set uncertainty propagation will not be
possible since propagation might need the uncertain data besides the
uncertainty.'

@property
def parent_nddata(self):

    message = u'uncertainty    is    not    associated    with    an    NDData    object'
    try:
        if (self._parent_nddata is None):
            raise MissingDataAssociationException(message)
        elif isinstance(self._parent_nddata, weakref.ref):
            return self._parent_nddata()
        else:
            log.info(u'parent_nddata    should    be    a    weakref    to    an    NDData    object.')
            return self._parent_nddata
    except AttributeError:
        raise MissingDataAssociationException(message)
