For Developers

FastAPI vs Flask: A Detailed Comparison

FastAPI vs Flask.


When you go to an e-commerce website and click on a button like Place Order, an HTTP request is sent to a backend and in the backend, there is a back-end server running which serves those HTTP requests. This web server can be written in a JavaScript framework like NodeJS or with a Python framework like Flask. But there is one more framework called FastAPI. In this article, we will learn about these two frameworks Flask and FastAPI.

Python is one of the most popular and widely used languages amongst developers. Be it Machine Learning, Deep Learning, Scripting, or API development, it is by far the most favored language. Python's popularity is because of the features and tools it offers like Flask, FastAPI, web-scraping, and many more. FastAPI and Flask are tools that are used by many machine learning developers or API developers. Flask came earlier so it is used by the majority of the people but FastAPI is also gaining that popularity. So, in this article, we will be understanding both these tools of Python, what are the features they offer to developers, and then we will have a comparison of their superpowers. Let's get started with understanding both of them so that a beginner can also understand.

What is Flask?

As we know Flask is a Python-based lightweight Web Server Gateway Interface (WSGI) web application framework. But what is a web framework? It is just a collection of modules and libraries that helps web app developers to write their applications hassle-free without thinking much about low-level details like protocol, thread management, etc. And WSGI is the specification of a common interface between web servers and web applications. Flask is also called a micro web framework because it does not require particular tools or libraries and aims to keep the core simple but extensible. It only provides the necessary components required for development such as routing, request handling, etc. It is very easy to get started with the flask application giving ease to beginners to start their development. You can use it for any kind of web application be it light or even complex ones too. Flask helps developers develop web applications like blogs, e-commerce websites, etc. It is also widely used for deploying machine learning models easily and conveniently. Flask is used in top tech companies also like: Netflix, Reddit, Mozilla, and so on.

Features of Flask:

  • Provides development server and debugger.
  • Light framework.
  • Uses Jinja2 templates.
  • Extensions are available for Flask that help enhance its functionalities.
  • Integrated support for unit testing is provided.
  • Neat API.

To install this in your system, use this command: pip install flask

You can refer to Flask documentation here:

What is FastAPI?

Flask is in use for ages and it is the most famous python framework for creating REST services. Indeed it is easy to use and deploy and is used effectively for creating microservices. Since Flask has also got its own disadvantages. So, to compensate for those issues of Flask, Python gave birth to a new framework called FastAPI. FastAPI is described as a modern and high-performance web framework for developing APIs with Python 3.6+. As the name says FastAPI is very fast. It offers very high performance on par with NodeJS and GO. To get started with FastAPI, you need to install FastAPI and uvicorn using pip. Uvicorn is an Asynchronous Server Gateway Interface server used for production. FastAPI has the advantage of handling requests asynchronously. All you have to do is while declaring endpoints put async keyword before that function. For example async def my_endpoint():, that's it.

Features of FastAPI:

  • FastAPI is considered one of the fastest frameworks of Python.
  • Its documentation is very simple, direct, and offers great editor support.
  • Because of its coding style, around 40% of induced bugs are reduced.
  • It is very much compatible with open standards for APIs and JSON schema.
  • Interactive API documentation.

You can refer to Flask documentation here:

So, now you know how good is FastAPI. Even many big companies like Uber, Netflix, Microsoft are using FastAPI to build their applications. Now, we know Flask and FastAPI and their features. Let's move on and discuss the differences between these two Python frameworks.

Differences between Flask and FastAPI


FastAPI does what it says on the tin. If you are experienced with languages like NodeJS or Go, you will find FastAPI's performance on par with that. Under the hood, FasAPI uses the asyncio library which allows developers to write concurrent code. Asyncio is of great help when doing tasks that involve waiting for something like fetching data from APIs, querying a database, reading the contents of a file. FastAPI implements ASGI specifications while Flask is constrained in a WSGI application.

Data validation:

No data validation is present in Flask, you can pass data of any type. Suppose input needed an integer and you gave it string, tuple, or list as an input, this will for sure lead to program crash. This validation in Flask can be handled explicitly by the developer but FastAPI gives this data validation as an inbuilt thing that makes a developer's life much easier.

Documentation support:

In FastAPI documentation is generated on the go when you are building the API. This is the very helpful and most requested thing from the developers. It generates a nice, clean User Interface where you can test your API even when you have written no code for UI. It is that easy and helpful. Refer to the below sample image to understand the same:


To access these automated generated docs, you just need to hit the endpoint with /docs or /redoc and Swagger UI will be presented to you which allows you to test the API endpoints. It lists all the endpoints you made in your application.

Community support:

Flask is an older framework and has rich community support, whereas FastAPI's community support is smaller as compared to other frameworks. This community support is of great help when you are stuck doing development. So, in Flask you will never ever face any such issue for finding answers to your bugs, but in FastAPI you may struggle a bit.

When to use Flask?

  • It is a battle tested framework, because it is very old and has good community support.
  • For web application development.
  • For developing a quick prototype.

When to use FastAPI?

  • For developing APIs from scratch.
  • If you want decreased number of bugs and errors in your code.
  • Good speed and performance as compared to flask.

Conclusion: Who is the winner?

Let's give this battle a final result, who is the winner? On one side, we have the very popular Flask and on the other side, we have got FastAPI which has won the hearts of users with so many built-in functionalities. We also know how simple and easy to use these two Python frameworks are. But FastAPI has got an upper edge over Flask. It is because of all the limitations FastAPI compensates for Flask.


Data validation in FastAPI is super awesome. This is very helpful while developing and debugging code that interacts with your API. The amazing in-built documentation support of FastAPI listed with all endpoints is like the cherry on top. FastAPI's runtime performance is amazing and it also takes less time to write code, fewer bugs, and many more such features. So, based on all these factors, I would suggest adopting FastAPI over Flask for your next REST project. However, you are free to choose any of the two frameworks depending upon your choices and use case but you can give a try to FastAPI as well and discover its hidden powers.



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