def _get_window_indexer(self) -> BaseIndexer:
"""
Return an indexer class that will compute the window start and end bounds
"""
return ExpandingIndexer()
def test_expanding_indexer():
s = Series(range(10))
indexer = ExpandingIndexer()
result = s.rolling(indexer).mean()
expected = s.expanding().mean()
tm.assert_series_equal(result, expected)
