FastAPI vs Flask

It has a simple but powerful dependency injection system, it can be used to handle authentication, per-user rate limiting, authorization controls (e.g. with roles), etc.

It is based on standards: OpenAPI, JSON Schema and OAuth 2.0.

It validates the data using the types you declared. Even in deeply nested JSON requests.

It's based on Starlette and Pydantic, so, it's one of the fastest Python frameworks.

It is based on Python type declarations, so, editors and tools can give great support. Including type checks and autocompletion everywhere.

It's easy to develop API based applications in Python on deadlines for Android and IOS Development.

It generates interactive API documentation automatically from your code.

It's independent of database or ORM, but compatible with all of them. Including relational databases and NoSQL.

It's based on Async IO, which gives it high concurrency. But you can use non-async libraries and it runs them appropriately.

It has some of the best documentation of any framework.

Because it's an async framework, it can handle async-native protocols like WebSockets.

It has integrated support for OAuth 2.0. Including declaring required scopes per endpoint. So, you can easily integrate it with external OAuth 2.0 providers or build your own with it.

Included support for background tasks, thanks to being based on Starlette.

Python's graphene library is included as an optional dependency meaning that GraphQL API's are supported out of the box, with no additional tweaking needed.

Flask is very easy to get up and going, with vanilla HTML or with bootstrap pieces. It doesn't take much lines of Python to load Flask to get headers working, etc, and since it's all modular you don't have to have something you don't want in your application.

Flask is one of the most popular Python web frameworks, if not the most popular one. As such, there's plenty of guides, tutorials, and libraries available for it. A large number of important Python libraries, such as SQLAlchemy have libraries for Flask, which add valuable bindings to make the development process and the integration between these libraries and Flask as easy as possible.

Even though it's pretty minimalistic out of the box, Flask still provides the necessary tools to build a quick prototype for a web app right after a fresh install. With all the main components pretty much packed in the flask package, building a simple web app in a single Python file is as easy as it gets.

Flask gives developers a lot of flexibility in how they develop their web applications.

For example, the choice of not having an ORM, but instead choosing one suited to the task, or another area where Flask gives a lot of options to developers is the templating. They can use Jinja2, Flask's default templating language or choose from a number of different templating languages they desire.

The official documentation is very thorough and complete. Everything is explained in-depth and followed by extremely well-explained tutorials that tackle real-world problems.