def METHOD_NAME(self, preprocessor):
    """Test that the checksum is computed for grade cells with different sources"""
    cell1 = create_grade_cell("a", "code", "foo", 1)
    cell1 = preprocessor.preprocess_cell(cell1, {}, 0)[0]
    cell2 = create_grade_cell("b", "code", "foo", 1)
    cell2 = preprocessor.preprocess_cell(cell2, {}, 0)[0]
    assert cell1.metadata.nbgrader["checksum"] == compute_checksum(cell1)
    assert cell2.metadata.nbgrader["checksum"] == compute_checksum(cell2)
    assert cell1.metadata.nbgrader["checksum"] != cell2.metadata.nbgrader["checksum"]