'Returns a list of XModuleDescriptor instances upon
which this module depends.'

def get_required_module_descriptors(self):

    descriptors = []
    for location in self.sources_list:
        try:
            descriptor = self.system.load_item(location)
            descriptors.append(descriptor)
        except ItemNotFoundError:
            msg = 'Invalid    module    by    location.'
            log.exception(msg)
            self.system.error_tracker(msg)
    return descriptors
