What happened after entered " flask run" on a terminal under the project directory?
flask is a Python script. Since you stated you are not a beginner, you should simply open the file (/usr/bin/flask) in your favorite text editor and start from there. There is no magic under the hood.
What happened after entered "flask run" on a terminal under the project directory?
How the python interpreter gets the file of flask.__main__.py and starts running project's code?
I know how Flask locates app. What I want to figure out is how command line instruction "flask run" get the flask/__main__.py bootup