'Get the type URIs for all attributes that have been marked
as required.
@returns: A list of the type URIs for attributes that have
been marked as required.
@rtype: [str]'

def getRequiredAttrs(self):

    required = []
    for (type_uri, attribute) in self.requested_attributes.iteritems():
        if attribute.required:
            required.append(type_uri)
    return required
