“(venv) $ export FLASK_APP=hello.py
(venv) $ export FLASK_DEBUG=1
(venv) $ flask run”
如下，当设置debug为1，允许reloader和debug两种模式，在reloader模式下，程序会自动检测code里的修改，非常方便的功能。
“(venv) $ flask run --help
Usage: flask run [OPTIONS]
Runs a local development server for the Flask application.
This local server is recommended for development purposes only but it can
also be used for simple intranet deployments. By default it will not
support any sort of concurrency at all to simplify debugging. This can be
changed with the --with-threads option which will enable basic
multithreading.
The reloader and debugger are by default enabled if the debug flag of
Options:
-h, --host TEXT The interface to bind to.
-p, --port INTEGER The port to bind to.
--reload / --no-reload Enable or disable the reloader. By default
the reloader is active if debug is enabled.
--debugger / --no-debugger Enable or disable the debugger. By default
the debugger is active if debug is enabled.
--eager-loading / --lazy-loader
Enable or disable eager loading. By default
eager loading is enabled if the reloader is
disabled.
--with-threads / --without-threads
Enable or disable multithreading.
--help Show this message and exit.”