为什么 assertRaises 不使用 python unittest 捕获我的属性错误？
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Why isn't assertRaises catching my Attribute Error using python unittest?
提问by Jeff
I'm trying to run this test: self.assertRaises(AttributeError, branch[0].childrennodes), and branch[0] does not have an attribute childrennodes, so it should be throwing an AttributeError, which the assertRaisesshould catch, but when I run the test, the test fails because it is throwing an AttributeError.
我正在尝试运行此测试：self.assertRaises(AttributeError, branch[0].childrennodes), 和branch[0] 没有属性childrennodes，因此它应该抛出一个AttributeError，assertRaises应该捕获它，但是当我运行测试时，测试失败，因为它抛出了一个AttributeError.
Traceback (most recent call last):
File "/home/tttt/../tttt/tests.py", line 504, in test_get_categories_branch
self.assertRaises(AttributeError, branch[0].children_nodes)
AttributeError: 'Category' object has no attribute 'children_nodes'
Any ideas?
有任何想法吗？
采纳答案by David Raznick
I think its because assert raises only accepts a callable. It evalutes to see if the callable raises an exception, not if the statement itself does.
我认为这是因为 assert raise 只接受一个可调用的。它评估可调用对象是否引发异常，而不是语句本身是否引发异常。
self.assertRaises(AttributeError, getattr, branch[0], "childrennodes")
should work.
应该管用。
EDIT:
编辑：
As THC4k correctly says it gathers the statements at collection time and will error then, not at testing time.
THC4k 正确地说它在收集时收集语句，然后会出错，而不是在测试时。
Also this is a reason why I like nose, it has a decorator (raises) that is useful and clearer for these kind of tests.
这也是我喜欢鼻子的一个原因，它有一个装饰器（加注），对这类测试有用且更清晰。
@raises(AttributeError)
def test_1(self)
branch[0].childrennodes
回答by Jonathan Hartley
When the test is running, before calling self.assertRaises, Python needs to find the value of all the method's arguments. In doing so, it evaluates branch[0].children_nodes, which raises an AttributeError. Since we haven't invoked assertRaises yet, this exception is not caught, causing the test to fail.
当测试运行时，在调用 self.assertRaises 之前，Python 需要找到所有方法参数的值。这样做时，它会评估branch[0].children_nodes，从而引发 AttributeError。由于我们还没有调用assertRaises，这个异常没有被捕获，导致测试失败。
The solution is to wrap branch[0].children_nodesin a function or a lambda:
解决方案是包装branch[0].children_nodes在函数或 lambda 中：
self.assertRaises(AttributeError, lambda: branch[0].children_nodes)
assertRaises can also be used as a context manager (Since Python 2.7, or in PyPI package 'unittest2'):
assertRaises 也可以用作上下文管理器（自 Python 2.7 起，或在 PyPI 包“unittest2”中）：
with self.assertRaises(AttributeError):
branch[0].children_nodes
# etc
This is nice because it can be used on arbitrary blocks of code in the middle of a test, rather than having to create a new function just to define the block of code to which it applies.
这很好，因为它可以在测试过程中用于任意代码块，而不必创建一个新函数来定义它适用的代码块。
It can give you access to the raised exception for further processing, if needed:
如果需要，它可以让您访问引发的异常以进行进一步处理：
with self.assertRaises(AttributeError) as cm:
branch[0].children_nodes
self.assertEquals(cm.exception.special_attribute, 123)
回答by Jonathan Hartley
pytest also has a similar decorator:
pytest 也有一个类似的装饰器：
from pytest import raises
def test_raising():
with raises(AttributeError):
branch[0].childrennodes
