unittest的源代码未显示assertRaises 的异常钩子：
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
因此，正如您所怀疑的那样，如果您想在继续保持assertRaises测试的同时截获异常，则可以形成自己的try / except块：
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
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