Here's the basic directory structure;

├── app
│   ├── config.py
│   ├── __init__.py
│   ├── models
│   │   └── __init__.py
│   ├── resources
│   │   └── __init__.py
│   ├── templates
│   │   └── index.html
│   ├── utils
│   │   └── __init__.py
│   └── views
│       └── __init__.py
├── README.md
├── requirements.txt
├── test
│   └── __init__.py
└── wsgi.py

models - This holds all your data classes and utility functions that interact with any kind of persitence (Mongo, Mysql etc)

resources - This package usually holds endpoints related to an API. Try to avoid putting view endpoints here. Put them in the view package instead

view - This package holds any endpoint that deals with any visual representation i.e endpoints that return html

templates - Put all your .html files here

utils - Put utility functions here. Mostly reusable logic used in the view and resources

config.py - All app your configuration goes here. Remember to put the right config in the right class. Anything reusable should go in Config

This is not a strict life or death way of structuring a flask app. Remember programming is art. As long as your app can be testable go with whatever makes you sleep at night 😌

Tips (Frequent updates expected):

1. For any file in the models package. Make sure it's within a class or method/function. It makes it easy to mock stuff during testing
2. While organizing your models be careful while using the orgranize imports feature available in text editors and IDEs it can affect how your models are loaded(i.e relationships) thus prevent your app from starting.
3. While testing your endpoints start with negative vibes first. Basically, start testing for non 200/201 responses first
4. Any method/function/class in the utils package should not depend on anything outside the utils package (Scope of your code. They can depend on libraries).
5. Make use of Flask extensions. They save a lot of time! Here is a good place to start. should be passed in as parameters. It helps prevent cyclic dependencies

Further References:

• Flask Project Layout
• Organizing your project
• Structure of a flask project