Flask "Error: The file/path provided does not appear to exist" although the file does exist
This message will occur if you issue flask run on the command line. Instead use python -m flask run after setting export FLASK_APP and export FLASK_ENV variables. I ran into this issue while following the Flask Tutorial when creating The Application Factory. The instruction does not specify to preface flask run with python -m.
I use export FLASK_APP=flask_app and then do flask run but I get the error:

Error: The file/path provided (flask_app) does not appear to exist.  Please verify the path is correct.  If app is not on PYTHONPATH, ensure the extension is .py

However, the file does exist and is even in the present working directory. Using the complete path to the file does not work either.