FastAPI is a modern, fast (high-performance), a web framework for building APIs with Python 3.7+ based on standard Python type hints. It is built on top of Starlette for the web parts and Pydantic for the data parts.

Flask is a lightweight Python web framework that provides useful tools and features for creating web applications. It is simple to use and provides a convenient way for developers to build APIs, websites, and applications quickly.

Here are some key differences between FastAPI and Flask:

  • Performance: FastAPI is designed to be very fast. It uses modern Python libraries and technology (such as asyncio and uvloop) to achieve very high performance. On the other hand, Flask is not designed to be as performant as FastAPI.
  • Async support: FastAPI is built on top of asyncio and supports async/await natively. This means you can use async/await syntax in your FastAPI application to write asynchronous code. Flask, on the other hand, does not have native async support. However, you can use libraries like aiohttp to add async capabilities to a Flask application.
  • Data validation: FastAPI has built-in data validation using Pydantic. It allows you to define the request and response payloads as Python data classes, and FastAPI will automatically validate the data based on the types and constraints defined in the data class. Flask does not have built-in data validation.
  • Automatic API documentation: FastAPI automatically generates API documentation based on your API code. It uses OpenAPI and JSON Schema to generate the documentation, which can be accessed via a Swagger UI or ReDoc. Flask does not have built-in API documentation generation.

Overall, FastAPI is a modern and high-performance framework for building APIs, while Flask is a lightweight and easy-to-use framework for building web applications and APIs. If you need a fast and reliable API, FastAPI is a good choice. If you want a simple and lightweight solution, Flask might be a better fit.