无法运行 README 中计算 BMI 的示例

[songquanpeng]

PyWebIO 版本：

PS D:\Temp\test-pywebio> pip show pywebio
Name: pywebio
Version: 1.0.1

Python 版本：

PS D:\Temp\test-pywebio> python --version
Python 3.8.3

报错信息：

PS D:\Temp\test-pywebio>  & 'C:\Users\admin\Anaconda3\python.exe' 'c:\Users\admin\.vscode\extensions\ms-python.python-2020.12.424452561\pythonFiles\lib\python\debugpy\launcher' '60851' '--' 'd:\Temp\test-pywebio\bmi.py'
Exception in thread Tornado-server:
Traceback (most recent call last): 
  File "C:\Users\admin\Anaconda3\lib\threading.py", line 932, in _bootstrap_inner
    self.run()
  File "C:\Users\admin\Anaconda3\lib\threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "C:\Users\admin\Anaconda3\lib\site-packages\pywebio\platform\tornado.py", line 296, in server_thread
    server, port = _setup_server(webio_handler=SingleSessionWSHandler, port=port, host='localhost')
  File "C:\Users\admin\Anaconda3\lib\site-packages\pywebio\platform\tornado.py", line 157, in _setup_server
    server = app.listen(port, address=host)
  File "C:\Users\admin\Anaconda3\lib\site-packages\tornado\web.py", line 2116, in listen
    server.listen(port, address)
  File "C:\Users\admin\Anaconda3\lib\site-packages\tornado\tcpserver.py", line 152, in listen
    self.add_sockets(sockets)
  File "C:\Users\admin\Anaconda3\lib\site-packages\tornado\tcpserver.py", line 165, in add_sockets
    self._handlers[sock.fileno()] = add_accept_handler(
  File "C:\Users\admin\Anaconda3\lib\site-packages\tornado\netutil.py", line 279, in add_accept_handler
    io_loop.add_handler(sock, accept_handler, IOLoop.READ)
  File "C:\Users\admin\Anaconda3\lib\site-packages\tornado\platform\asyncio.py", line 100, in add_handler
    self.asyncio_loop.add_reader(fd, self._handle_events, fd, IOLoop.READ)
  File "C:\Users\admin\Anaconda3\lib\asyncio\events.py", line 501, in add_reader
    raise NotImplementedError
NotImplementedError

[wang0618]

这应该是python3.8 asyncio 在 windows 上默认使用的事件循环与Tornado(PyWebIO默认使用Tornado作为后端Server)不兼容造成的，见 tornadoweb/tornado#2804 。

可以尝试在脚本开始处加入以下代码：

import sys
import asyncio

if sys.platform == 'win32':
    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

[songquanpeng]

it works, thx~