FastAPI vs Flask: Key Differences, Performance, and Use Cases

If you are starting to build web apps and APIs in Python, you will hear about Flask and FastAPI frameworks that dominate the web development landscape in Python. While both frameworks serve the purpose of creating web applications and APIs, they differ significantly in performance, design philosophy, and developer experience. This article compares the features, advantages, and disadvantages of Flask and FastAPI frameworks to help you understand their design philosophy and use cases.

What is FastAPI?

FastAPI is a modern, fast, and high-performance framework for building application programming interfaces (APIs). You can install FastAPI on your system using the following command:

pip install fastapi 

Copy to clipboard
Copy to clipboard

The latest FastAPI version requires Python 3.8 and above. FastAPI is popular for API development due to the following features:

• Speed and performance: FastAPI is one of the fastest frameworks in Python. It is built for performance and provides comparable performance to NodeJS.
• Code style: FastAPI is intuitive and also provides great autocomplete features. This reduces development time and helps avoid bugs caused by human errors.
• Compatibility: FastAPI is fully compatible with OpenAPI and JSON schema.
• Documentation: FastAPI provides interactive API documentation. Its documentation is simple and has great editor support, which helps you get started with FastAPI easily.
• Asynchronous support: FastAPI is built on Starlette, which is a very lightweight and high-performance Asynchronous Server Gateway Interface (ASGI) toolkit.

Due to its features, FastAPI is used by companies like Uber, Microsoft, Hugging Face, Shopify, Airbnb, and ByteDance (TikTok) to develop internal tools and consumer products.

FastAPI was built by borrowing the concept of minimalism and simplicity from Flask, another web framework. Let’s briefly discuss Flask’s features.

What is Flask?

Flask is a lightweight micro-framework that we can use to build web applications in Python. It doesn’t require setting up any libraries or frameworks and has minimal overhead. You can install Flask on your machine using the following command:

pip install Flask 

Copy to clipboard
Copy to clipboard

The latest Flask version requires Python 3.9 and above. It has the following features:

• Micro framework: Flask is a micro-framework and helps us build a simple but extensible web app. It doesn’t come with built-in tools like form validation or database abstraction, which we can add later if required.
• Jinja 2 templating engine: Flask comes with Jinja2, which is a fast and extensible templating engine for rendering dynamic HTML.
• Built-in tools: Flask also provides a built-in development server, debugger, and integrated tools for unit testing.
• Modular architecture: Flask allows us to build applications that are structured and flexible by dividing them into modules.
• Extensibility: Flask helps us keep our web apps simple and extensible. After building the core components, we can enhance the functionalities of our apps using different extensions.

Due to its simplicity and features, multiple companies like Reddit, Netflix, Pinterest, LinkedIn, Spotify, and Mozilla use Flask in their tech stack.

Now that we have a brief understanding of FastAPI and Flask, let’s discuss their advantages and disadvantages.

Advantages and disadvantages of Flask

Flask has various advantages due to its design principles and focus on simplicity.

• Easy to learn: Flask is one of the easiest web frameworks for beginners. It is ideal for teaching web development concepts without overwhelming the learners with framework-specific details.
• Minimalism: Flask is a micro-framework. It provides essential functionalities like routing and request/response handling, leaving everything else to the developers. We have complete control over the structure, libraries, and components we want to use in the web application.
• Flexibility: Flask doesn’t enforce a particular project layout or tooling. We can build very small apps or very large and complex systems by experimenting quickly with different architectures, without the need to follow a specific pattern. Flask lets us choose our own ORM, database layer, validation libraries, authentication, and other functionalities based on our needs and preferences.
• Large community and ecosystem: Flask has been around for almost fifteen years and enjoys a great ecosystem. There are a lot of plugins, tutorials, and extensions that you can use to build web apps. You can also use Flask-specific developer forums like StackOverflow, dev.to, and reddit to find solutions to your problems when your project runs into an issue.
• Good for prototyping and MVPs: Flask is lightweight and ideal for quickly building prototypes, MVPs, or simple APIs without spending time on setup. It also has a built-in development server and debugger that helps you easily develop web apps.

Despite all the advantages due to minimalism and simplicity, Flask also has a few limitations:

• Manual development for large apps: Flask is a minimalistic framework. To build a large web application, we need to organize the project structure manually and handle dependency injection, validation, authentication, etc. If we don’t follow software development practices and version control, Flask apps can become very hard to maintain.
• Not async by default: Flask is a web server gateway interface (WSGI) compliant, built for synchronous request handling. We can implement asynchronous request handling with newer versions and third-party tools, but it’s not first-class like in FastAPI. Due to this, Flask apps can be slower than FastAPI when processing concurrent requests.
• Limited built-in tools: Flask doesn’t come with an ORM, form validation, or authentication system. It also doesn’t provide built-in security features like CSRF protection, XSS protection, etc. We need to install and configure extensions manually to implement these functionalities.
• No auto-generated documentation: Flask does not automatically generate documentation for the APIs. To enable automatic documentation, we need to integrate additional tools like Flask-RESTX or Flask-Swagger manually.

Now that we know the advantages and disadvantages of Flask, let’s discuss the same for FastAPI.

Advantages and disadvantages of FastAPI

FastAPI has various advantages due to its design principles and emphasis on modern Python features, performance, and automatic validation.

• Speed: FastAPI is designed to help us quickly build reliable and secure APIs. With FastAPI, you can create APIs that are fast both in performance and development time.
• Data validation: FastAPI uses type hints and Pydantic to perform automatic data validation. This helps us build clean, maintainable, and robust code.
• Automatic API documentation: FastAPI automatically generates docs using Swagger UI and ReDoc. Automatic documentation helps us create and maintain fully browsable API docs without any extra effort, increasing the code’s maintainability.
• Security: With FastAPI, we also get easy-to-use tools for handling authentication, authorization, OAuth2, and JSON web tokens using just a very few lines of code.
• Concurrency: FastAPI provides support for asynchronous processing using async/await methods. This helps us build scalable APIs that can handle many concurrent requests.
• Production-ready code: FastAPI is designed for real-world use with features like cross-origin resource sharing, request/response parsing, background tasks, WebSockets, etc. We can easily productionize APIs created with FastAPI using ASGI servers like Uvicorn.

With all the advantages of FastAPI, there are certain disadvantages, too.

• Learning curve: FastAPI uses asynchronous programming, and misusing async/await functionalities can cause performance issues. Thus, you need to understand concurrency and async concepts well.
• Dependency on type hints: FastAPI uses type hints for data validation. If you are uncomfortable writing precise type annotations in your code, you might find development slower or frustrating initially.
• Lack of certain functionalities: FastAPI is a comparatively new framework that is still growing. Some features, such as the admin dashboard and session management, that exist for frameworks like Flask aren’t available in FastAPI. Similarly, the number of third-party plugins is also less than the number of plugins available for Flask.

Now that we have discussed the features, advantages, and disadvantages of Flask and FastAPI, let’s compare these frameworks.

FastAPI vs Flask: Comparison

Flask and FastAPI find their uses in building small APIs to real-time recommendation tools. Let’s compare FastAPI vs. Flask for some use cases.

Flask vs. FastAPI: speed and performance

Due to its native support for asynchronous request/response handling, FastAPI is better than Flask for API-heavy and high-concurrency workloads in terms of speed and performance. For example, a FastAPI app using async and Uvicorn can handle around 20,000+ requests per second in benchmarks. On the same hardware, a Flask app using Gunicorn might handle around 4,000 to 5,000 requests per second.

FastAPI vs. Flask for API development

FastAPI is better than Flask for building a production-grade RESTful API with type hints, auto-generated docs, and high concurrency. If you are quickly prototyping or deploying a small project that won’t need asynchronous request handling, Flask should be the go-to choice, as it is easier to develop apps using Flask.

Flask vs. FastAPI: Popularity

Flask is more popular among web developers as it is an older and very mature framework. On the other hand, FastAPI is also gaining popularity among developers who build API-centric applications. If we look at Google search trends in the past five years, Flask has completely dominated the web search volume. However, FastAPI shows a steeper upward trend, indicating growing interest among developers.

If we use GitHub stars as a measure of popularity, the FastAPI GitHub repository seems to be more popular than the Flask GitHub repository, with twenty percent more stars. Overall, FastAPI is popular for API-centric applications, whereas Flask is popular for building web apps and dashboards.

FastAPI vs. Flask for machine learning

If you are building an API-first machine learning service, such as model-as-a-service or real-time recommendation, FastAPI should be the first choice. It provides better support for concurrency and async I/O operations, and will make your machine-learning applications more scalable.

If you are building a simple ML model that takes web form input and returns a prediction, Flask should be your go-to choice. Flask also helps you render dashboards along with serving your model. Hence, it is easier to build MVPs and prototypes using Flask.

Having discussed the features, advantages, disadvantages, and use cases for Flask and FastAPI, let’s summarize them in a table for more clarity.

Key differences: FastAPI vs Flask

Conclusion

Both Flask and FastAPI are powerful tools for building web APIs in Python. Flask is simple and flexible, making it ideal for quick prototypes and traditional web apps. FastAPI, on the other hand, excels in performance, scalability, and modern API development.

To learn more about developing web apps, you can take this Learn Flask course. You might also like this course on API Development with Swagger and OpenAPI that discusses how to develop APIs using Swagger tooling and the OpenAPI specification.

Frequently asked questions

1. Is FastAPI enough for the backend?

Yes, FastAPI can fully handle backend development, especially for applications focused on APIs, microservices, or machine learning inference. It supports routing, middleware, authentication, database integration, and other backend tasks. However, if your project requires server-rendered pages, an admin interface, or a full-stack framework out of the box, Flask or Django might be more suitable.

2. Is Flask good for API development?

Yes, Flask is suitable for API development, especially for small to medium-sized applications or when you need to quickly build and iterate on an idea. It offers simplicity, flexibility, and a large ecosystem of extensions for handling routing, request parsing, and database integration.

3. Is FastAPI good for microservices?

FastAPI is an excellent choice for microservices due to its high performance and native support for asynchronous programming. FastAPI’s automatic validation, dependency injection, and built-in documentation process simplify development and integration between services. Its compatibility with ASGI servers like Uvicorn also ensures efficient handling of concurrent requests, which is critical in microservice architectures.

4. Is FastAPI faster than Spring Boot?

FastAPI is generally faster than Spring Boot in terms of raw API response time and throughput, especially for I/O-bound tasks. FastAPI processes requests in an asynchronous manner using async/await. In contrast, Spring Boot handles requests synchronously, which can introduce more overhead.

5. Is Flask free for commercial use?

Yes, Flask is free for commercial use. It is released under the BSD-3-Clause license, a permissive open-source license. We can use, modify, and distribute Flask in both open-source and commercial applications without paying fees or releasing the source code.