def METHOD_NAME() -> None:
    dt1 = DataType(nullable=True)
    dt2 = DataType(nullable=False)
    dt3 = DataType(nullable=True)
    assert dt1 is dt3
    assert dt1 is not dt2
    assert len(DataType.__instances__) == 2
    del dt3
    assert len(DataType.__instances__) == 2
    del dt1
    assert len(DataType.__instances__) == 1
    del dt2
    assert len(DataType.__instances__) == 0