To run the Flask and Django application : I don't understand why if I want to run Flask application I need    

    (venv) $ export FLASK_APP=microblog.py
    (venv) $ flask run

But if I want to run Django application I only    

    (venv) $ python manage.py runserver

without `export DJANGO_APP=microblog.py`   
Why? Why I need the export app in the first case, but in the second case I don't need?