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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 supports async. APIFlask has 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

APIFlask is a framework

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 it makes possible to overwrite and change the internal behavior of Flask. For example:

  • Rewrite Flask.dispatch_request to ensure the natural order of the arguments injected into the view function (APIFlask 1.x).
  • Add route shortcuts to the Flask and the Blueprint class (added in Flask 2.0).
  • Rewrite Flask.make_response to support returning list as JSON response (added in Flask 2.2).
  • Rewrite Flask.add_url_rule to support generating OpenAPI spec for class-based views.

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

  • 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.