def test_drop_obs_column():
    adata = ad.AnnData(np.array(X_list), obs=obs_dict, dtype="int32")

    subset = adata[:2]
    assert subset.is_view
    # returns a copy of obs
    assert subset.obs.drop(columns=["oanno1"]).columns.tolist() == ["oanno2", "oanno3"]
    assert subset.is_view
    # would modify obs, so it should actualize subset and not modify adata
    subset.obs.drop(columns=["oanno1"], inplace=True)
    assert not subset.is_view
    assert subset.obs.columns.tolist() == ["oanno2", "oanno3"]

    assert adata.obs.columns.tolist() == ["oanno1", "oanno2", "oanno3"]
def union(matches, root, cwd):
    """Union a list of matchers.

    If the list is empty, return nevermatcher.
    If the list only contains one non-None value, return that matcher.
    Otherwise return a union matcher.
    """
    matches = list(filter(None, matches))
    if len(matches) == 0:
        return nevermatcher(root, cwd)
    elif len(matches) == 1:
        return matches[0]
    else:
        return unionmatcher(matches)