'Late binding callback.
This method is called on this field descriptor when the target RDFValue
class is finally defined. It gives the field descriptor an opportunity to
initialize after the point of definition.
Args:
target: The target nested class.
Raises:
TypeError: If the target class is not of the expected type.'

def LateBind(self, target=None):

    if (not issubclass(target, RDFProtoStruct)):
        raise TypeError(('Field    %s    expects    a    protobuf,    but    target    is    %s' % self), target)
    self.late_bound = False
    self.type = target
    self.owner.AddDescriptor(self)
