启动 flask 时报错：OSError: [Errno 48] Address already in use
应该是有谁占用了 5000 端口。解决方法：
lsof -i tcp:5000，找到占用此端口的进程号kill {进程号}，杀掉进程- 再次启动 flask
完整实例：
(venv) ➜ flaskr lsof -i tcp:5000
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
Python 48998 plough 3u IPv4 0xce1d99ccf653b655 0t0 TCP localhost:commplex-main (LISTEN)
(venv) ➜ flaskr kill 48998
(venv) ➜ flaskr lsof -i tcp:5000
(venv) ➜ flaskr flask run
* Serving Flask app "flaskr"
* Forcing debug mode on
* Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
