Python 3:如何测试异常？

Question

我在测试异常时遇到了问题，这些异常将在Anwithinpython3.4中引发。我只是不能让测试运行来保证代码的安全：

import logging
...
class Foo(object):
    ...
    def foo(self, src, dst):
        try:
            with pysftp.Connection(self._host, username=self._username, password=self._password) as connection:
                connection.put(src, dst)
                connection.close()
        except (
                ConnectionException,
                CredentialException,
                SSHException,
                AuthenticationException,
                HostKeysException,
                PasswordRequiredException
        ) as e:
            self._log.error(e)

这就是我要测试的方法：

import logging
...
class TestFoo(TestCase):
    @parameterized.expand([
        ('ConnectionException', ConnectionException),
        ('CredentialException', CredentialException),
        ('SSHException', SSHException),
        ('AuthenticationException', AuthenticationException),
        ('HostKeysException', HostKeysException),
        ('PasswordRequiredException', PasswordRequiredException),
    ])
    @patch('pysftp.Connection', spec_set=pysftp.Connection)
    def test_foo_exceptions(self, _, ex, sftp_mock):
        """
        NOTE: take a look at:
              http://stackoverflow.com/questions/37014904/mocking-python-class-in-unit-test-and-verifying-an-instance
              to get an understanding of __enter__ and __exit__
        """
        sftp_mock.return_value = Mock(
            spec=pysftp.Connection,
            side_effect=ex,
            __enter__ = lambda self: self,
            __exit__ = lambda *args: None
        )
        foo = Foo('host', 'user', 'pass', Mock(spec_set=logging.Logger))
        foo.foo('src', 'dst')
        self.assertEqual(foo._log.error.call_count, 1)

但是它失败了--输出：

Failure
...
AssertionError: 0 != 1

Answer 1

您的sftp_mock.return_value对象从未被调用，因此不会触发side_effect，也不会引发异常。只有当pysftp.Connection(...)的返回值本身再次被调用时，才会调用它。

将副作用直接设置在模拟上。

sftp_mock.side_effect = ex

请注意，现在pysftp.Connection(...)表达式会引发异常，该表达式的返回值在with语句中用作上下文管理器不再重要。

请注意，您的异常会抱怨没有获得任何参数；传入异常实例，而不是类型：

@parameterized.expand([
    ('ConnectionException', ConnectionException('host', 1234)),
    # ... etc.
])