'Validate the I{value} is of the correct class.
@param value: The value to validate.
@type value: any
@raise AttributeError: When I{value} is invalid.'

def validate(self, value):

    if (value is None):
        return
    if (len(self.classes) and (not isinstance(value, self.classes))):
        msg = ('"%s"    must    be:    %s' % (self.name, self.classes))
        raise AttributeError, msg
