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APIFlask is a lightweight Python web API framework based on Flask and marshmallow-code projects. It's easy to use, highly customizable, ORM/ODM-agnostic, and 100% compatible with the Flask ecosystem. It starts as a fork of APIFairy and is inspired by flask-smorest and FastAPI (see Comparison and Motivations for the comparison between these projects).

With APIFlask, you will have:

  • More sugars for view function (@input(), @output(), @app.get(), and more)
  • Automatic request validation and deserialization (with Webargs)
  • Automatic response formatting and serialization (with Marshmallow)
  • Automatic OpenAPI Specification (OAS, formerly Swagger Specification) document generation (with APISpec)
  • Automatic interactive API documentation (with Swagger UI and Redoc)
  • API authentication support (with Flask-HTTPAuth)
  • Automatic JSON response for HTTP errors

Currently this project is in active development stage, bugs and breaking changes are expected. Welcome to leave any suggestions or feedbacks in this issue or just submit a pull request to improve it. Thank you!


  • Python 3.7+
  • Flask 1.1.0+


For Linux and macOS:

$ pip3 install apiflask

For Windows:

> pip install apiflask


from apiflask import APIFlask, Schema, input, output, abort
from apiflask.fields import Integer, String
from apiflask.validators import Length, OneOf

app = APIFlask(__name__)

pets = [
        'id': 0,
        'name': 'Kitty',
        'category': 'cat'
        'id': 1,
        'name': 'Coco',
        'category': 'dog'

class PetInSchema(Schema):
    name = String(required=True, validate=Length(0, 10))
    category = String(required=True, validate=OneOf(['dog', 'cat']))

class PetOutSchema(Schema):
    id = Integer()
    name = String()
    category = String()

def say_hello():
    # returning a dict equals to use jsonify()
    return {'message': 'Hello!'}

def get_pet(pet_id):
    if pet_id > len(pets) - 1:
    # you can also return an ORM model class instance directly
    return pets[pet_id]

def update_pet(pet_id, data):
    # the parsed input data (dict) will be injected into the view function
    if pet_id > len(pets) - 1:
    data['id'] = pet_id
    pets[pet_id] = data
    return pets[pet_id]

Save the file as, then run it with:

$ flask run

Now visit the interactive API documentation (Swagger UI) at http://localhost:5000/docs:

Or you can visit the alternative API documentation (Redoc) at http://localhost:5000/redoc:

The auto-generated OpenAPI spec file is available at http://localhost:5000/openapi.json.

For a more complete example, see /examples.

Relationship with Flask

APIFlask is a thin wrapper on top of Flask, you only need to remember three differences:

  • When creating an application instance, use APIFlask instead of Flask.
  • When creating a blueprint instance, use APIBlueprint instead of Blueprint.
  • The abort() function from APIFlask (apiflask.abort) returns JSON error response.

For a minimal Flask application:

from flask import Flask, request, escape

app = Flask(__name__)

def hello():
    name = request.args.get('name', 'Human')
    return f'Hello, {escape(name)}'

Now change to APIFlask:

from apiflask import APIFlask  # step one
from flask import request, escape

app = APIFlask(__name__)  # step two

def hello():
    name = request.args.get('name', 'Human')
    return f'Hello, {escape(name)}'

In a word, to make Web API development in Flask more easily, APIFlask provides APIFlask and APIBlueprint to extend Flask's Flask and Blueprint objects, and it also ships with some helpful utilities. Other than that, you are actually using Flask.