如何捕获 gen.Task 内的异常？

Question

我在 python 2.7，龙卷风 4.5

以下代码无效：未触发 except 块。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，我可以在对 gen.Task 的调用周围捕获 co_do_thing 中的异常；但是我不知道我如何调用 maybe_throw 的上下文。在我的例子中，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更有意义。

我是否只需要重构它以在较低级别调用 gen.Task？那会很烦人:/

Answer 1

根据我的测试，它似乎可以工作，但出现了异常。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()

测试通过：

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

OK

这里是 q.py，我添加了 return 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")

如何捕获 gen.Task 内的异常？

How to catch exceptions inside gen.Task?

python

tornado