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Comparison and Motivations

APIFlask starts as a fork of APIFairy (which share similar APIs with flask-smorest) and is inspired by flask-smorest and FastAPI. So, what are the differences between APIFlask and APIFairy/flask-smorest/FastAPI?

In a word, I try to provide an elegant (act as a framework, no need to instantiate additional extension object) and simple (more automation support for OpenAPI/API documentation) solution for creating web APIs with Flask. Here is a summary of the differences between APIFlask and similar projects.

APIFlask vs FastAPI

  • For the web part, FastAPI builds on top of Starlette, while APIFlask builts on top of Flask.
  • For the data part (serialization/deserialization, OpenAPI support), FastAPI relies on Pydantic, while APIFlask uses marshmallow-code projects (marshmallow, webargs, apispec).
  • APIFlask builds on top of Flask, so it's compatible with Flask extensions.
  • FastAPI support async. APIFlask will have the basic async support with Flask 2.0.
  • APIFlask provides more decorators to help organize things better.
  • FastAPI injects the input data as an object, while APIFlask passes it as a dict.
  • APIFlask has built-in class-based views support based on Flask's MethodView.
  • On top of Swagger UI and Redoc, APIFlask supports more API documentation tools: Elements, RapiDoc, and RapiPDF.

APIFlask vs APIFairy/flask-smorest

It's a framework (and why?)

Although APIFlask is a thin wrapper on top of Flask, it's actually a framework. Thus, there is no need to instantiate additional extension object:

from flask import Flask
from flask_api_extension import APIExtension

app = Flask(__name__)
api = APIExtension(app)

You only need to use the APIFlask class to replace the Flask class:

from apiflask import APIFlask

app = APIFlask(__name__)

The key reasons behind making APIFlask a framework instead of a Flask extension is:

  • I have to rewrite the Flask class to ensure the natural order of the arguments injected into the view function.
  • I have to rewrite the Flask and the Blueprint class to add route shortcuts.
  • Possible to add more high-level features, such as support returning list as JSON response.

See the following two sections for more details.

A natural order of view arguments

By acting as a framework on top of Flask, APIFlask can overwrite the way Flask passes path arguments to view functions from keyword arguments to positional arguments.

Assume a view like this:

@app.get('/<category>/articles/<int:article_id>')  # category, article_id
@app.input(ArticleQuery, location='query')  # query
@app.input(ArticleIn)  # data
def get_article(category, article_id, query, data):

With APIFlask, you can accept the arguments in the view function in a natural way (from left to right, from top to bottom):

def get_article(category, article_id, query, data)

However, with APIFairy, Flask-Smorest, or webargs, the path variables (category and article_id) need to be declared after the input data:

def get_article(query, data, category, article_id)

Route shortcuts

APIFlask added some route shortcuts (app.get(), , etc) for app.route(..., methods=['GET/POST...']).

Instead of doing something like this:

@app.route('/pets', methods=['POST'])
def create_pet():

You can just use'/pets')
def create_pet():


Flask will have original support for these route shortcuts in the 2.0 version.

More automation for OpenAPI generating

  • Add an auto-summary for the view function based on the name of view functions.
  • Add success response (200) for a bare view function that only uses route decorators.
  • Add validation error response (400) for view functions that use input decorator.
  • Add authentication error response (401) for view functions that use auth_required decorator.
  • Add 404 response for view functions that contain URL variables.
  • Add response schema for potential error responses of view function passed with doc decorator. For example, doc(responses=[404, 405]).
  • etc.


These automation behaviors can be changed with related configuration variables.

More features compare with APIFairy

  • Add a doc decorator to allow set the OpenAPI spec for view functions in an explicit way.
  • Support more OpenAPI fields (all fields from info, servers, response/requestBody/parameters example, path deprecated, etc).
  • Support to customize the API documentation config and CDN URLs.
  • Return JSON response for all HTTP errors and Auth errors as default.
  • Class-based view support.
  • etc.