在Python的assertRaises中使用上下文管理器

Question

Python的文档提到，unittest中的assertRaises()方法可以作为上下文管理器使用。下面的代码给出了一个来自Python文档的简单示例。在testsample()方法中调用assertRaises()是没问题的。

现在我想在异常被抛出时访问这个异常，但如果我把这部分注释掉，改为使用上下文管理器的代码块，就会在执行时遇到AttributeError: __exit__的错误。这在Python 2.7.2和3.2.2中都会发生。我可以在try...except块中捕获这个异常，按那种方式访问它，但文档似乎暗示上下文管理器也能做到这一点。

我这里是不是还有其他地方做错了呢？

class TestSequenceFunctions(unittest.TestCase):

    def setUp(self):
        self.seq = [x for x in range(10)]

    def testshuffle(self):
        # make sure the shuffled sequence does not lose any elements
        random.shuffle(self.seq)
        self.seq.sort()
        self.assertEqual(self.seq, [x for x in range(10)])

    def testchoice(self):
        element = random.choice(self.seq)
        self.assert_(element in self.seq)

    def testsample(self):
        self.assertRaises(ValueError, random.sample, self.seq, 20)

        # with self.assertRaises(ValueError, random.sample, self.seq, 20):
        #     print("Inside cm")

        for element in random.sample(self.seq, 5):
            self.assert_(element in self.seq)

if __name__ == '__main__':
    unittest.main()

Answer 1

根据文档的说明：

如果调用时没有提供 callableObj，或者提供的是 None，那么将返回一个上下文对象。

所以那段代码应该是：

with self.assertRaises(ValueError):
    random.sample(self.seq, 20)

Answer 2

看起来还没有人提过这个：

import unittest
# For python < 2.7, do import unittest2 as unittest

class Class(object):
    def should_raise(self):
        raise ValueError('expected arg')

class test_Class(unittest.TestCase):
    def test_something(self):
        DUT = Class()
        with self.assertRaises(ValueError) as exception_context_manager:
            DUT.should_raise()
        exception = exception_context_manager.exception

        self.assertEqual(exception.args, ('expected arg', ))

我通常用 e_cm 来表示异常上下文管理器。

Answer 3

unittest的源代码里没有显示出assertRaises的异常处理机制：

class _AssertRaisesContext(object):
    """A context manager used to implement TestCase.assertRaises* methods."""

    def __init__(self, expected, test_case, expected_regexp=None):
        self.expected = expected
        self.failureException = test_case.failureException
        self.expected_regexp = expected_regexp

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, tb):
        if exc_type is None:
            try:
                exc_name = self.expected.__name__
            except AttributeError:
                exc_name = str(self.expected)
            raise self.failureException(
                "{0} not raised".format(exc_name))
        if not issubclass(exc_type, self.expected):
            # let unexpected exceptions pass through
            return False
        self.exception = exc_value # store for later retrieval
        if self.expected_regexp is None:
            return True

        expected_regexp = self.expected_regexp
        if isinstance(expected_regexp, basestring):
            expected_regexp = re.compile(expected_regexp)
        if not expected_regexp.search(str(exc_value)):
            raise self.failureException('"%s" does not match "%s"' %
                     (expected_regexp.pattern, str(exc_value)))
        return True

所以，正如你所猜测的，如果你想在保持assertRaises测试的同时捕获异常，自己写一个try/except块是个不错的选择：

def testsample(self):
    with self.assertRaises(ValueError):
         try:
             random.sample(self.seq, 20)
         except ValueError as e:
             # do some action with e
             self.assertEqual(e.args,
                              ('sample larger than population',))
             # now let the context manager do its work
             raise