Flask vs FastAPI: Detailed Comparison

Flask and FastAPI are both Python web frameworks that can be used to build web applications and APIs. However, there are some key differences between the two frameworks.

1. HTTP Methods

Flask

Flask is a micro framework and provides support for all HTTP methods which are GET, POST, PUT, DELETE, etc. You can use HTTP methods in Flask using decorators. This makes route handling straightforward. Following are how GET and POST methods are used in Flask.

@app.route(“/”, methods = [“GET”])
@app.route(“/”, methods = [“POST”])

FastAPI

FastAPI has more modern Python features like type hints and asynchronous support. This asynchronous support makes it easy to handle asynchronous operations and I/O-bound tasks. Following are how GET and POST methods are used in FastAPI.

@app.get(“/”)
@app.post(“/”)

2. Data Validation

• Flask: Flask does not support in-built data validation. If you try to pass the wrong type of input than the desired input type, the program will simply crash. Developers can use external libraries like WTForms or some other libraries to ensure data validation.
• FastAPI: FastAPI on the other hand provides in-built automatic data validation. FastAPI achieves it as it is integrated with Pydantic models. These models use Python-type annotations which help validate the requests and responses to and from the API.

3. Error Message Display

• Flask: Flask uses custom error handlers that have to be defined by the developers to display custom error messages. By default, HTML pages are used to display error messages in Flask.
• FastAPI: FastAPI which is integrated with Pydantic models automatically generates detailed and user-friendly error messages. By default, FastAPI displays error messages in JSON format.

4. Asynchronous Tasks

• Flask: Flask does not support asynchronous tasks but requires external resources for it. Due to its non-inherent design for asynchronous programming, the chances to achieve high performance using asynchronous tasks become difficult.
• FastAPI: FastAPI ASGI supports asynchronous tasks. Due to its inherent design for asynchronous programming, FastAPI can handle large volumes of concurrent requests and is ideal to be used for real-time applications.

5. Performance

• Flask: Flask performs well in applications that do not require I/O bound tasks and applications that do not involve handling large volumes of concurrent connections.
• FastAPI: FastAPI outperforms Flask in applications that require I/O bound tasks and applications that involve handling large volumes of concurrent connections.

6. Documentation Support

• Flask: Flask provides manual documentation support. To automate the documentation process of a Flask application, external tools like Swagger have to be used.
• FastAPI: FastAPI provides in-built automatic documentation support. It helps provide a UI for testing your service. To access these automatically generated documentation, hit the endpoint of the API that has to be tested with /docs.

7. Community Support

• Flask: Flask being an older framework has great community support. It has many resources, from tutorials and guides to third-party extensions and plugins.
• FastAPI: FastAPI is a newer framework with a small community size which is gradually increasing due to its great performance and features.

Let’s understand the difference between Flask and FastAPI using a real-world analogy. Imagine you are hosting a party where Flask and FastAPI are the two methods using which you serve your guests. Flask is like a buffet-style approach where all the options are laid on the table and each individual chooses the service they want to use. Your role is to prepare the food or the backend logic for the application and serve it and then it is up to the guests or the clients how they wish to interact with them.

On the other hand, FastAPI is like a catering service where you provide a menu of options and guests place their orders. FastAPI or the catering service takes care of all the behind-the-scenes tasks and ensures to provide a fast and efficient experience to the clients. In this article, we’ll discuss the differences between Flask and FastAPI and which one to choose, leaving the choice to the user to decide which framework suits them for their application needs.