def METHOD_NAME(df_factory):
    x = np.array([np.nan, 0, 1, np.nan, 2, np.nan], dtype='f4')
    df = df_factory(x=x)
    assert dropnan(set(df.x.unique(dropnan=True))) == {0, 1, 2}
    assert dropnan(set(df.x.unique()), expect=1) == {0, 1, 2}