Source code: Lib/asyncio/tasks.py
Source code: Lib/asyncio/coroutines.py
18.5.3.1. Coroutines
Coroutines used with asyncio may be implemented using the async def statement, or by using generators. The async def type of coroutine was added in Python 3.5, and is recommended if there is no need to support older Python versions.
与asyncio一起使用的协程可以使用async def语句或使用generators。在Python 3.5中添加了协程的async def类型，如果不需要支持旧的Python版本，则推荐使用
Generator-based coroutines should be decorated with @asyncio.coroutine, although this is not strictly enforced. The decorator enables compatibility with async def coroutines, and also serves as documentation. Generator-based coroutines use the yield from syntax introduced in PEP 380, instead of the original yield syntax.
基于生成器的协程应该用@asyncio.coroutine来装饰，尽管这不是严格执行的。装饰器能够与async def协程兼容，并且还作为文档。基于生成器的协程使用在 PEP 380中引入的产生 来自 而不是原始的yield语法。
The word “coroutine”, like the word “generator”, is used for two different (though related) concepts:
“协程”一词与“生成器”一样，用于两个不同的（虽然相关的）概念：
The function that defines a coroutine (a function definition using async def or decorated with @asyncio.coroutine). If disambiguation is needed we will call this a coroutine function (iscoroutinefunction() returns True).
The object obtained by calling a coroutine function. This object represents a computation or an I/O operation (usually a combination) that will complete eventually. If disambiguation is needed we will call it a coroutine object (iscoroutine() returns True).
定义协程的函数（使用async def或用@asyncio.coroutine装饰的函数定义） 。如果需要消除歧义，我们称之为协程函数（iscoroutinefunction()返回True）。
通过调用协程函数获得的对象。此对象表示将最终完成的计算或I / O操作（通常是组合）。如果需要消除歧义，我们将它称为协程对象（iscoroutine()返回True）
Things a coroutine can do:
协程可以做的事：
result = await future or result = yield from future – suspends the coroutine until the future is done, then returns the future’s result, or raises an exception, which will be propagated. (If the future is cancelled, it will raise a CancelledError exception.) Note that tasks are futures, and everything said about futures also applies to tasks.
result = await future or result = yield from future – suspends the coroutine until the future is done, then returns the future’s result, or raises an exception, which will be propagated. （如果未来被取消，则会引发CancelledError异常。）注意任务是 futures，关于 futures 的一切也适用于任务。
result = await coroutine or result = yield from coroutine – wait for another coroutine to produce a result (or raise an exception, which will be propagated). The coroutine expression must be a call to another coroutine.
result = await coroutine或result = yield from coroutine - 等待另一个协程产生结果或引发异常，其将被传播）。coroutine表达式必须是到另一个协程的调用。
return expression – produce a result to the coroutine that is waiting for this one using await or yield from.
raise exception – raise an exception in the coroutine that is waiting for this one using await or yield from.
Calling a coroutine does not start its code running – the coroutine object returned by the call doesn’t do anything until you schedule its execution. There are two basic ways to start it running: call await coroutine or yield from coroutine from another coroutine (assuming the other coroutine is already running!), or schedule its execution using the ensure_future() function or the AbstractEventLoop.create_task() method.
Coroutines (and tasks) can only run when the event loop is running.
@asyncio.coroutine
Decorator to mark generator-based coroutines. This enables the generator use yield from to call async def coroutines, and also enables the generator to be called by async def coroutines, for instance using an await expression.
There is no need to decorate async def coroutines themselves.
If the generator is not yielded from before it is destroyed, an error message is logged. See Detect coroutines never scheduled.
Note In this documentation, some methods are documented as coroutines, even if they are plain Python functions returning a Future. This is intentional to have a freedom of tweaking the implementation of these functions in the future. If such a function is needed to be used in a callback-style code, wrap its result with ensure_future().