问题描述
在Python 3.6中，我可以在协程内部使用yield.但是我不能使用yield from.
In Python 3.6, I am able to use yield inside a coroutine. However I am not able to use yield from.
下面是我的代码.在第3行中，我等待另一个协程.在第4行上，我尝试yield from一个文件.为什么Python 3.6不允许我这样做?
Below is my code. On line 3 I await another coroutine. On line 4 I try to yield from a file. Why won't Python 3.6 allow me to do that?
async def read_file(self, filename):
with tempfile.NamedTemporaryFile(mode='r', delete=True, dir='/tmp', prefix='sftp') as tmp_file:
await self.copy_file(filename, tmp_file)
yield from open(tmp_file)
这是Python 3.6为上述代码引发的异常:
Here's the exception Python 3.6 raises for the above code:
File "example.py", line 4
yield from open(tmp_file)
^
SyntaxError: 'yield from' inside async function
推荐答案
根据 PEP 525 ，它在Python 3.6中引入了异步生成器:
According to PEP 525, which introduces asyncronous generators in Python 3.6:
虽然在理论上可以实现对以下内容的yield from支持 异步发电机，则需要认真重新设计 生成器的实现.
While it is theoretically possible to implement yield from support for asynchronous generators, it would require a serious redesign of the generators implementation.
yield from对于异步生成器也不太重要，因为 无需提供实现另一种机制的机制 协程之上的协程协议.并组成异步 生成器可以使用一个简单的async for循环:
yield from is also less critical for asynchronous generators, since there is no need provide a mechanism of implementing another coroutines protocol on top of coroutines. And to compose asynchronous generators a simple async for loop can be used:
async def g1():
yield 1
yield 2
async def g2():
async for v in g1():
yield v
如您所见，答案归结为实施起来太困难了，反正您也不需要它."
As you can see, the answer boils down to "it would be too hard to implement, and you don't need it anyway".
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