asyncio 的 call_later 引发“生成器”对象不能用协程对象调用

Question

我有一些使用 Python 3.4 的 asyncio 使用 call_later 编写的简单代码。代码应该打印，等待 10 秒，然后再次打印（而是在应该执行 end() 时引发 TypeError，见下文）：

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    asyncio.get_event_loop().call_later(10, end())

@asyncio.coroutine
def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

给出错误：

Exception in callback <generator object coro at 0x7fc88eeaddc8>()
handle: <TimerHandle when=31677.188005054 <generator object coro at 0x7fc88eeaddc8>()>
Traceback (most recent call last):
  File "/usr/lib64/python3.4/asyncio/events.py", line 119, in _run
    self._callback(*self._args)
TypeError: 'generator' object is not callable

据我从文档 (https://docs.python.org/3/library/asyncio-task.html#coroutine) 中得知，call_later 采用协程对象，该对象是通过调用协程函数获得的。这似乎是我所做的，但 asyncio 没有正确调用 end()。

这应该怎么做？

Answer 1

call_later 旨在接受回调（即常规函数对象），而不是协程。较新版本的 Python 实际上会明确说明这一点：

Starting to wait.
Task exception was never retrieved
future: <Task finished coro=<coro() done, defined at /usr/lib/python3.4/asyncio/coroutines.py:139> exception=TypeError('coroutines cannot be used with call_at()',)>
Traceback (most recent call last):
  File "/usr/lib/python3.4/asyncio/tasks.py", line 238, in _step
    result = next(coro)
  File "/usr/lib/python3.4/asyncio/coroutines.py", line 141, in coro
    res = func(*args, **kw)
  File "aio.py", line 6, in begin
    asyncio.get_event_loop().call_later(10, end())
  File "/usr/lib/python3.4/asyncio/base_events.py", line 392, in call_later
    timer = self.call_at(self.time() + delay, callback, *args)
  File "/usr/lib/python3.4/asyncio/base_events.py", line 404, in call_at
    raise TypeError("coroutines cannot be used with call_at()")
TypeError: coroutines cannot be used with call_at()

要使您的代码正常工作，end 需要是一个常规函数，然后您将其传递给 call_later：

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    asyncio.get_event_loop().call_later(10, end)

def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

输出：

Starting to wait.
completed
Goodbye!

如果end 需要成为协程，延迟后调用它的更自然的方法是使用asyncio.sleep：

import asyncio

@asyncio.coroutine
def begin():
    print("Starting to wait.")
    yield from asyncio.sleep(10)
    yield from end()

@asyncio.coroutine
def end():
    print("completed")

if __name__ == "__main__":
    try:
        loop = asyncio.get_event_loop()
        loop.create_task(begin())
        loop.run_forever()
    except KeyboardInterrupt:
        print("Goodbye!")

虽然从技术上讲，这确实有效：

asyncio.get_event_loop().call_later(10, lambda: asyncio.async(end()))