如何使assertRaises（）打印在Python 2.7中传递的* args，** wargs？

Question

我在这样的循环中使用assertRaises ：

for i in ['a', 'b', 'c']: 
    self.assertRaises(ValidationError, my_method, i)

问题是每当测试失败时，输出将如下所示：

  File "/bla/test.py", line 4, in test_assertion
      my_method(i)
AssertionError: ValidationError not raised

测试失败时如何打印出i的值？ 像这样：

  File "/bla/test.py", line 4, in test_assertion
      my_method('b')
AssertionError: ValidationError not raised

Answer 1

这是subTests上下文管理器的理想情况。 但是，这仅在python 3中可用。我认为最好的解决方案是创建自己的subTests版本。 好处是可以很容易地设置subTests的基本模拟，并且如果将subTests重新移植到python 2，则将很容易切换。

import unittest
import sys
from contextlib import contextmanager

class TestCaseWithSubTests(unittest.TestCase):

    @contextmanager
    def subTest(self, **kwargs):
        try:
            yield None
        except:
            exc_class, exc, tb = sys.exc_info()
            kwargs_desc = ", ".join("{}={!r}".format(k, v) for k, v in kwargs.items())
            new_exc = exc_class("with {}: {}".format(kwargs_desc, exc))
            raise exc_class, new_exc, tb.tb_next

class MyTest(TestCaseWithSubTests):

    def test_with_assertion_error(self):
        for i in [0, 1]:
            with self.subTest(i=i), self.assertRaises(ZeroDivisionError):
                1 / i

    def test_with_value_error(self):
        def f(i):
            raise ValueError

        for i in [0, 1]:
            with self.subTest(i=i), self.assertRaises(ZeroDivisionError):
                f(i)


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

产生以下输出：

FE
======================================================================
ERROR: test_with_value_error (__main__.MyTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\dunes\py2sub_tests.py", line 30, in test_with_value_error
    f(i)
  File "C:\Python27\lib\contextlib.py", line 35, in __exit__
    self.gen.throw(type, value, traceback)
  File "C:\Users\dunes\py2sub_tests.py", line 30, in test_with_value_error
    f(i)
  File "C:\Users\dunes\py2sub_tests.py", line 26, in f
    raise ValueError
ValueError: with i=0: 

======================================================================
FAIL: test_with_assertion_error (__main__.MyTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "C:\Users\dunes\py2sub_tests.py", line 22, in test_with_assertion_error
    1 / i
  File "C:\Python27\lib\contextlib.py", line 35, in __exit__
    self.gen.throw(type, value, traceback)
  File "C:\Users\dunes\py2sub_tests.py", line 22, in test_with_assertion_error
    1 / i
AssertionError: with i=1: ZeroDivisionError not raised

----------------------------------------------------------------------
Ran 2 tests in 0.006s

FAILED (failures=1, errors=1)

Answer 2

一种解决方案是使用assertRaises()作为上下文管理器。

with self.assertRaises(ValidationError):
    my_method(i)
    print("Failed when i was",i)

仅当my_method()行成功通过时，print语句才会执行，因此当测试失败时，它将显示为输出。 有点my_method() ，但确实有效（除非my_method()抛出不是ValidationError错误）。