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In Development Last updated: December 6, 2022
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FastAPI and Flask are Python frameworks you will come across when creating web applications.

Python is one of the most popular programming languages, as you can use it for backend development, data science, and app development.

Developers love Python as they can employ different programming styles, such as functional, object-oriented, and reflective, to various web/software applications. 


A web framework is a collection of functions, modules, libraries, and classes that helps developers create applications without thinking a lot about thread and protocol management. Django and Ruby on Rails are examples of frameworks

On the other hand, a micro framework is a small framework that provides only essential services.

FastAPI is classified as a framework, while Flask falls under micro frameworks. Ruby Sinatra is another example of micro frameworks. 

As a developer, you may have already come across multiple frameworks and micro web frameworks, and deciding which suits your needs best may not be that easy. Let us explain in detail how FastAPI and Flask frameworks work:

What is FastAPI?

FastAPI is a Python web framework for creating Rest APIs and fast web applications. The web framework was released in 2018 and is supported by Python 3.6 and newer versions. 

FastAPI lives up to its name as it is fast, and its high performance is comparable to that of GO and NodeJS. Top-tier companies like Uber and Netflix already use the FastAPI framework across their apps. 

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Key features of FastAPI

  • Dependency injection. FastAPI has an inbuilt dependency injection that ensures classes are not directly dependent on each other. This feature makes it easy to make code changes, and code modularity increases, which makes it easy to scale. 
  • Concurrency. FastAPI allows out-of-the-box concurrency, which means you avoid async/await management or creating event loops. 
  • Inbuilt validation. You no longer have to validate using external tools, as FastAPI has an inbuilt validation capability. This web framework detects invalid data types and will return the errors in JSON format. Data validation is done on Pydantic library, greatly reducing errors during code writing. 

Pros of FastAPI 👍

  • Fast. FastAPI is one of the fastest Python frameworks. You can also use FastAPI to create APIs even with limited programming knowledge. 
  • Built-in monitoring tools. FastAPI comes with inbuilt monitoring tools that can provide alerts when you reach certain thresholds. 
  • Easy to adapt. FastAPI uses a toolkit-based approach, so you don’t have to create everything from scratch. You can thus use various boilerplates to create powerful APIs. 

Cons of FastAPI 👎

  • Lacks inbuilt security system. FastAPI lacks an inbuilt security system but uses module. 
  • Still new. FastAPI is still in development, and there are a lot of things that can be improved. Even if its community is growing, it is still not comparable to other Python frameworks. 

What is Flask?

Flask is a lightweight, open-source Python micro framework with a simple and extensible core. Flask does not need any particular libraries or tools to work. This microframework provides the necessary components for web development, such as request handling and routing. 

This framework was released in 2010 and is based on Werkzeug and Jinja2. Flask supports REST applications using extensions such as Flask-RESTful, Flask-RESTPlus, and Flask-Classful. You can use Flask to create social media bots, static sites, and eCommerce platforms. Some notable companies using Flask in their applications are Netflix, Lyft, and Zillow. 

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Key features of Flask

  • Built-in server and a fast debugger. You can simulate the production environment through the in-built server in Flask. You thus don’t have to put your app on production to debug.
  • Uses Jinja2 templates. Jinja2 is an expressive, fast, and extensible templating engine. The templates have special placeholders that allow developers to write code similar to Python syntax. 
  • Light web framework. Flask is lightweight but a highly scalable web framework. You can also add new features on the go, saving time. 
  • Integrated support for unit testing. Unit testing is available on Flask. You can thus simulate different scenarios and easily test your app.
  • Great community. Flask has been around since 2010 and is highly documented and known for its awesome community. 

Pros of Flask 👍

  • Flexible. As a developer, Flask gives you control over how you want your app to behave and look. 
  • Scalable. If you want to create a large application, you do not have to worry, as Flask allows you to do that with minimal effort. 
  • Has many 3rd party libraries. You can do so much more using third-party libraries compatible with flask.

Cons of Flask 👎

  • Complex framework. It might be hard to understand how Flask works, and you need some programming background to use it. 
  • Slow. When compared to FastAPI, Flask is somehow slower. 

FastAPI Vs. Flask: A comparison of the features

Data Validation

FastAPI comes with the Pydantic library for data validation, making it easier to validate data. The developer can thus declare validation and extra information on the parameters they have when using FastAPI.

Flask, on the other hand, lacks validation. For instance, when the program needs an integer as the input, but you give a string, the program will definitely crash. The developer has to take care of validation when working with Flask. You can still use Flask-Marshmallow extensions or Flask-Inputs on the community library for data validation in Flask. However, such libraries are developed separately from the main framework and may bring compatibility issues. 

Documentation Support 

Documentation is very important as it guides other users using your API. Documentation is generated as you build your API using FastAPI. The web framework also produces a clean user interface (UI), and you can even test the API even if you haven’t yet written the front-end code. 

You can easily access the automated docs after hitting the endpoints with /docs or /redoc and Swagger UI while using FastAPI. The docs will also list all the endpoints of your API. 

Flask lacks automatic documentation. Even though it has documentation support, you can only write it manually. 


Flask uses Web Server Gateway Interface (WSGI), a Python standard that was created to enable servers and web applications to interface with each other. WSGI can be somewhat challenging for beginners but somehow easier to crack for developers coming from Ruby or PHP. 

The creators of FastAPI introduced Asynchronous Server Gateway Interface (ASGI). With ASGI, you can create event-driven, asynchronous web applications. ASGI is a standard interface between async-capable Python web servers, applications, and frameworks. 

You can mount apps from WSGI frameworks like Flask on top of a FastAPI app. Such an environment allows you to have a FastAPI app on the root folder and have Flask (WSGI) handling requests on different paths. 

Displaying error messages 

FastAPI displays messages in JSON format. On the other hand, the Flask framework displays error messages using HTML pages. 

Community support 

Flask was created in 2010 and has attracted a robust community over the years. FastAPI was created in 2018, and its community is not that big. A sizable community is handy when you get stuck during coding, as you can always ask questions. A big community is also likely to contribute a lot towards creating new features that can be used in the framework. 

Data ValidationNo validationIn-built validation
CommunityHas a strong communityHas a small community
Error message displayDisplayed in HTML pagesDisplayed in JSON format
Documentation supportManualAutomatic
Asynchronous task supportNoYes
PerformanceConstrained in WCGI Uses ASGI
FastAPI vs. Flask

When to use FastAPI over Flask and vice versa

FastAPI and Flask are both awesome frameworks but suitable for different use cases. Even though Flask is a micro-framework, it is worth noting that FastAPI is built on it. 

Flask is a micro framework, while FastAPI is a full-stack framework with all the tools you need to create an API.

FastAPI is suitable when creating APIs. FastAPI has various options for creating backend servers quickly, even if you are a beginner in coding. If speed and performance are important to you, FastAPI will be perfect. It is also an awesome choice if you want to create a CDN and you anticipate that there will be a lot of traffic. 

On the other hand, Flask has various tools and is ideal when you want to prototype new ideas and applications. It is also a good choice when you want to create a microservice with a few endpoints. Flask will also come in handy if you want to create a small application that is likely to grow/expand in ways you may not have figured out yet. 

When to use FastAPI?

  • Create web APIs
  • Create APIs with fewer bugs and errors

When to use Flask?

  • Create prototypes 
  • Create web applications

Wrapping Up 

The choice of whether to use FastAPI or Flask for your Python app will depend on your needs and preferences. If you want to create a high-performance web API, then FastAPI is an awesome choice. On the other hand, if you want to create a quick prototype or a microservice, it is advisable to go the Flask way.

  • Titus Kamunya
    Titus is a Software Engineer and Technical Writer. He develops web apps and writes on SaaS, React, HTML, CSS, JavaScript, Ruby and Ruby on Rails read more
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