Flas app has a config attribute which is a dict object and its used to set app configurations for flask.
To read more about configuration handling in flask, click here.
Flask config can be set via environment variable using from_envvar method. In this case we are using FLASK_SETTINGS environment variable
Flask can be run via inline env vars, for example:
FLASK_SETTINGS=settings_module/dev.py flask run
There multiple ways of loading config. More details on it can be found here.