记录个小问题
错误如下：

 * Serving Flask app "hello" (lazy loading)
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: off
Traceback (most recent call last):
  File "hello.py", line 10, in <module>
    app.run( host='0.0.0.0' )
  File "/root/ray/lib/python3.5/site-packages/flask/app.py", line 943, in run
    run_simple(host, port, self, **options)
  File "/root/ray/lib/python3.5/site-packages/werkzeug/serving.py", line 814, in run_simple
    inner()
  File "/root/ray/lib/python3.5/site-packages/werkzeug/serving.py", line 774, in inner
    fd=fd)
  File "/root/ray/lib/python3.5/site-packages/werkzeug/serving.py", line 660, in make_server
    passthrough_errors, ssl_context, fd=fd)
  File "/root/ray/lib/python3.5/site-packages/werkzeug/serving.py", line 577, in __init__
    self.address_family), handler)
  File "/usr/local/python3/lib/python3.5/socketserver.py", line 443, in __init__
    self.server_bind()
  File "/usr/local/python3/lib/python3.5/http/server.py", line 138, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/usr/local/python3/lib/python3.5/socketserver.py", line 457, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

提示说地址被占用。

输入命令：

jobs -l

原来，有个应用并没有关闭

再输入一下命令：

kill %n (n代表你的flask应用序号)

完成以上操作可以重新开启应用了。