call site 3 for test.collect.Directory.listchain
test/testing/test_config.py - line 411
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   def test_get_collector_trail_topdir_and_beyond(self):
       config = py.test.config._reparse([self.tmpdir])
       col = config._getcollector(config.topdir)
       trail = config.get_collector_trail(col)
       assert len(trail) == 2
       assert trail[0] == '.'
       assert trail[1] == ()
       col2 = config._getcollector(trail)
       assert col2.fspath == config.topdir
->     assert len(col2.listchain()) == 1
       col3 = config._getcollector(config.topdir.dirpath())
       py.test.raises(ValueError, 
             "config.get_collector_trail(col3)")