如何捕获gen.任务中的异常？

Question

我使用的是python 2.7和tornado 4.5

以下代码不起作用：除块不被触发。我不明白为什么？

@gen.coroutine
def co_do_thing():
  yield gen.Task(do_thing)

def do_thing(callback):
  try:
    a, b = ...
    result = maybe_throw(a, b, callback)
  except Exception as e:
    # this block is not called
    if a:
      raise ApiError("called with A")
    elif b:
      raise ApiError("called with B")
    else:
      raise e

def maybe_throw(arg1, arg2, callback):
  if random.random() < 0.5:
    raise AssertionError("yikes")
  callback("done")

相反，我可以捕获gen调用周围的co_do_thing中的异常。任务；但是我没有我如何调用maybe_throw的上下文。在我的例子中，更有意义的是maybe_throw引发一个较低级别的异常，并且调用者根据输入将其转换为人类可读的错误。

我是否只需要重构它来调用较低级别的gen.Task？那会很烦人：/

Answer 1

当我测试它似乎工作，一个例外被提出。下面是简单的测试套件：

import q  # q.py is the file with question's code

import unittest
from mock import patch, Mock
from tornado.testing import gen_test, AsyncTestCase


class MyTest(AsyncTestCase):

    def setUp(self):
        self.mock_random = patch('q.random').start()
        AsyncTestCase.setUp(self)

    def tearDown(self):
        AsyncTestCase.tearDown(self)
        patch.stopall()

    @gen_test
    def test_no_error(self):
        self.mock_random.return_value = 0.7
        res = yield q.co_do_thing()
        self.assertEqual(res, 'done')

    @gen_test
    def test_exception(self):
        self.mock_random.return_value = 0.1
        with self.assertRaises(Exception) as ctx:
            yield q.co_do_thing()
        self.assertEqual(ctx.exception.message, 'called with A')


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

通过的测试：

..
----------------------------------------------------------------------
Ran 2 tests in 0.002s

OK

这里是q.py，我添加了返回语句来测试它。

from random import random
from tornado import gen

@gen.coroutine
def co_do_thing():
    res = yield gen.Task(do_thing)
    # added: to enable to test it meaningfully
    raise gen.Return(res)

def do_thing(callback):
    try:
        a, b = 22, 33
        result = maybe_throw(a, b, callback)
    except Exception as e:
        if a:
            raise Exception("called with A")
        elif b:
            raise Exception("called with B")
        else:
            raise e

def maybe_throw(arg1, arg2, callback):
    if random() < 0.5:
        raise AssertionError("yikes")
    callback("done")