Refer to the following file structure for a better understanding:

flask_app/
    - run.py
    - my_app/
        – __init__.py
        - hello/
            - __init__.py
            - models.py
            - views.py

First, the flask_app/run.py file will look something like the following lines of code:

from my_app import app
app.run(debug=True)

Then, the flask_app/my_app/__init__.py file will look something like the following lines of code:

from flask import Flask
app = Flask(__name__)

import my_app.hello.views

Then, we will have an empty file just to make the enclosing folder a Python package, flask_app/my_app/hello/__init__.py:

# No content.
# We need this file just to make this folder a python module.

The models file, flask_app/my_app/hello/models.py, has a non-persistent key-value store:

MESSAGES = {
    'default': 'Hello to the World of Flask!',
}

Finally, the following is the views file, flask_app/my_app/hello/views.py. Here, we fetch the message corresponding to the key that is asked for and also have a provision to create or update a message:

from my_app import app
from my_app.hello.models import MESSAGES

@app.route('/')
@app.route('/hello')
def hello_world():
    return MESSAGES['default']


@app.route('/show/<key>')
def get_message(key):
    return MESSAGES.get(key) or "%s not found!" % key


@app.route('/add/<key>/<message>')
def add_or_update_message(key, message):
    MESSAGES[key] = message
    return "%s Added/Updated" % key

We can see that we have a circular import between my_app/__init__.py and my_app/hello/views.py, where, in the former, we import views from the latter, and in the latter, we import the app from the former. So, this actually makes the two modules depend on each other, but here, it is actually fine as we won't be using views in my_app/__init__.py. We do the import of views at the bottom of the file so that they are not used anyway.

We have used a very simple non-persistent in-memory key-value store for the demonstration of the model layout structure. It is true that we could have written the dictionary for the MESSAGES hash map in views.py itself, but it's best practice to keep the model and view layers separate.

So, we can run this app using just run.py:

$ python run.py
* Running on http://127.0.0.1:5000/
* Restarting with reloader

Now, we can see that we have already defined a default message in MESSAGES. We can view this message by opening http://127.0.0.1:5000/show/default. To add a new message, we can type http://127.0.0.1:5000/add/great/Flask%20is%20greatgreat!!. This will update the MESSAGES key-value store to look like the following:

MESSAGES = {
    'default': 'Hello to the World of Flask!',
    'great': 'Flask is great!!',
}

Now, if we open the link http://127.0.0.1:5000/show/great in a browser, we will see our message, which, otherwise, would have appeared as a not-found message.