class determine_test_args(object):
    implements(IRenderable)
    def getRenderingFor(props):
      if props.hasProperty('TRIGGER_TESTS'):]
          tests=props['TRIGGER_TESTS']
          if isinstance(tests, str):
              return '--tests=%s'%props['TRIGGER_TESTS']
          else:
              return '--tests=%s' % (','.join(tests))
      else:
          return []

b.addStep(ShellCommand(command=['run-tests', determine_test_args()))