pip install flask-apispec

from flask import Flask
from flask_apispec import use_kwargs, marshal_with

from marshmallow import Schema
from webargs import fields

from .models import Pet

app = Flask(__name__)

class PetSchema(Schema):
    class Meta:
        fields = ('name', 'category', 'size')

@app.route('/pets')
@use_kwargs({'category': fields.Str(), 'size': fields.Str()})
@marshal_with(PetSchema(many=True))
def get_pets(**kwargs):
    return Pet.query.filter_by(**kwargs)

from flask import make_response
from flask_apispec.views import MethodResource

class PetResource(MethodResource):

    @marshal_with(PetSchema)
    def get(self, pet_id):
        return Pet.query.filter(Pet.id == pet_id).one()

    @use_kwargs(PetSchema)
    @marshal_with(PetSchema, code=201)
    def post(self, **kwargs):
        return Pet(**kwargs)

    @use_kwargs(PetSchema)
    @marshal_with(PetSchema)
    def put(self, pet_id, **kwargs):
        pet = Pet.query.filter(Pet.id == pet_id).one()
        pet.__dict__.update(**kwargs)
        return pet

    @marshal_with(None, code=204)
    def delete(self, pet_id):
        pet = Pet.query.filter(Pet.id == pet_id).one()
        pet.delete()
        return make_response('', 204)

from apispec import APISpec
from apispec.ext.marshmallow import MarshmallowPlugin
from flask_apispec.extension import FlaskApiSpec

app.config.update({
    'APISPEC_SPEC': APISpec(
        title='pets',
        version='v1',
        plugins=[MarshmallowPlugin()],
    ),
    'APISPEC_SWAGGER_URL': '/swagger/',
})
docs = FlaskApiSpec(app)

docs.register(get_pets)
docs.register(PetResource)