相关疑难解决方法(0)

yieldPython中关键字的用途是什么？它有什么作用？

例如,我试图理解这段代码¹:

def _get_child_candidates(self, distance, min_dist, max_dist):
    if self._leftchild and distance - max_dist < self._median:
        yield self._leftchild
    if self._rightchild and distance + max_dist >= self._median:
        yield self._rightchild  

这是来电者:

result, candidates = [], [self]
while candidates:
    node = candidates.pop()
    distance = node._get_dist(obj)
    if distance <= max_dist and distance >= min_dist:
        result.extend(node._values)
    candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
return result

_get_child_candidates调用该方法时会发生什么？列表是否返回？单个元素？它又被召唤了吗？后续通话何时停止？

_{1.代码来自Jochen Schulz(jrschulz),他为度量空间创建了一个很棒的Python库.这是完整源代码的链接:模块mspace.}

假设我有一个可以传递的异步迭代,async for然后如何将它映射并过滤到新的异步迭代器？以下代码是对我如何使用同步迭代执行相同操作的修改不起作用,因为s中yield不允许这样做async def.

async def mapfilter(aiterable, p, func):
    async for payload in aiterable:
        if p(payload):

            # This part isn't allowed, but hopefully it should be clear
            # what I'm trying to accomplish.
            yield func(payload)

我正在尝试将此Python2.7代码重写为新的异步世界顺序:

def get_api_results(func, iterable):
    pool = multiprocessing.Pool(5)
    for res in pool.map(func, iterable):
        yield res

map()阻塞直到计算完所有结果,所以我试图将其重写为异步实现,一旦准备就会产生结果.同样map(),返回值必须按照与之相同的顺序返回iterable.我试过这个(我需要requests因为传统的身份验证要求):

import requests

def get(i):
    r = requests.get('https://example.com/api/items/%s' % i)
    return i, r.json()

async def get_api_results():
    loop = asyncio.get_event_loop()
    futures = []
    for n in range(1, 11):
        futures.append(loop.run_in_executor(None, get, n))
    async for f in futures:
        k, v = await f
        yield k, v

for r in get_api_results():
    print(r)

但是使用Python 3.6我得到了:

  File "scratch.py", line 16, in <module>
    for r in …

我有一个像这样的阻塞,非异步代码:

def f():
    def inner():
        while True:
            yield read()
    return inner()

使用此代码,调用者可以选择何时停止函数以生成数据.如何将此更改为异步？此解决方案不起作用:

async def f():
    async def inner():
        while True:
            yield await coroutine_read()
    return inner()

...因为 yield不能用于async def功能.如果我async从inner()签名中删除,我不能再使用await了.

我希望运行一个模拟，同时在一个 plot 中输出它的进度。我一直在查看很多线程和多处理的示例，但它们都非常复杂。所以我认为使用 Python 的新asyncio库应该会更容易。

我找到了一个例子（How to use 'yield' inside async function?）并为我的原因修改了它：

import matplotlib.pyplot as plt
import asyncio
import numpy as np


class DataAnalysis():
    def __init__(self):
        # asyncio so we can plot data and run simulation in parallel
        loop = asyncio.get_event_loop()
        try:
            loop.run_until_complete(self.plot_reward())
        finally:
            loop.run_until_complete(
                loop.shutdown_asyncgens())  # see: https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.AbstractEventLoop.shutdown_asyncgens
            loop.close()

    async def async_generator(self):
        for i in range(3):
            await asyncio.sleep(.4)
            yield i * i

    async def plot_reward(self):
        # Prepare the data
        x = …

我正在尝试为websockets客户端学习异步。我尝试的每一段代码都会出现以下错误：

RuntimeError：无法从正在运行的事件循环中调用asyncio.run（）

我尝试了最简单的代码，它始终会给出RuntimeError。我尝试再次安装完整的anaconda发行版，等等，但找不到问题所在。

我正在将Spyder 3.3.3与Python 3.7.3结合使用

可以正常工作的代码示例：

import asyncio

async def main():
    print('hello')
    await asyncio.sleep(1)
    print('world')

asyncio.run(main())

错误信息：

File "C:\Users\jmart\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 786, in runfile
  execfile(filename, namespace)
File "C:\Users\jmart\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 110, in execfile
  exec(compile(f.read(), filename, 'exec'), namespace)
File "C:/Users/jmart/Documents/asynk2.py", line 8, in <module>
  asyncio.run(main())
File "C:\Users\jmart\Anaconda3\lib\asyncio\runners.py", line 34, in run
  "asyncio.run() cannot be called from a running event loop")
RuntimeError: asyncio.run() cannot be called from a running event loop