表驱动测试中如何正确使用assertRaises并避免重复调用被测函数

在编写表驱动测试时，存在部分测试用例会抛出异常、部分不会的场景，开发者希望避免在两处重复调用被测函数my_code。直接向assertRaises传入None的方式会导致测试失败，当前采用的分支判断方案存在重复调用my_code的问题，最终通过在基础测试用例中添加辅助函数assertRaisesUnlessNone实现了目标。

初始尝试代码 #

尝试直接给assertRaises传None，但测试失败：

tests = [
   (0, None),
   (1, None),
   (-1, TooFewException),
   (99, None),
   (100, TooManyException),
]
for n, exc in tests:
    with self.assertRaises(exc):
        results = my_code(n)
        assert len(results) == n

现有分支方案代码 #

通过分支判断区分异常和正常场景，但存在my_code重复调用的问题：

tests = [
   (0, None),
   (1, None),
   (-1, TooFewException),
   (99, None),
   (100, TooManyException),
]
for n, exc in tests:
    if exc is not None:
        with self.assertRaises(exc):
            my_code(n)
    else:
        results = my_code(n)
        assert len(results) == n

优化后代码 #

在基础测试用例中实现assertRaisesUnlessNone辅助函数，统一逻辑，避免重复调用：

tests = [
   (0, None),
   (1, None),
   (-1, TooFewException),
   (99, None),
   (100, TooManyException),
]
for n, exc in tests:
    with self.assertRaisesUnlessNone(exc):
        results = my_code(n)
        assert len(results) == n

内容的提问来源于stack exchange，提问作者Thomas David Baker