from flasgger import Swagger, SwaggerView, swag_from
from flask import Flask, jsonify, request, redirect
from marshmallow import Schema, fields

app = Flask(__name__)
swag = Swagger(app)


class User(Schema):
    username = fields.Str(required=True, default="Sirius Black")
    # wrong default "180" to force validation error
    age = fields.Int(required=True, min=18, default="180")
    tags = fields.List(fields.Str(), default=["wizard", "hogwarts", "dead"])


class UserPostView(SwaggerView):
    tags = ['users']
    parameters = User
    responses = {
        200: {
            'description': 'A single user item',
            'schema': User
        }
    }
    validation = True

    def post(self):
        return jsonify(request.json)


app.add_url_rule(
    '/schemevalidation',
    view_func=UserPostView.as_view('schemevalidation'),
    methods=['POST']
)


@app.route('/schemevalidation2', methods=['POST'])
@swag_from(
    dict(
        tags=["users"],
        parameters=User,
        responses={
            200: {
                'description': 'A single user item',
                'schema': User
            }
        }
    ),
    validation=True
)
def route():
    return jsonify(request.json)


@app.route('/')
def index():
    return redirect('/apidocs')


if __name__ == '__main__':
    app.run()

Traceback (most recent call last):
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flasgger/utils.py", line 271, in wrapper
    **validate_args
  File "/Projects/PycharmProjects/oa/venv/lib/python3.7/site-packages/flasgger/utils.py", line 394, in validate
    item for item in swag.get('parameters', [])
TypeError: 'SchemaMeta' object is not iterable
127.0.0.1 - - [25/Feb/2021 00:50:25] "POST /schemevalidation2 HTTP/1.1" 500 -

definitions = {}
specs.update(convert_schemas(specs, definitions))
specs['definitions'] = definitions
....

 if isinstance(specs, dict):
        # Add code for converting a schema
        definitions = {}
        specs.update(convert_schemas(specs, definitions))
        specs['definitions'] = definitions

        set_from_specs_dict(function)
        validate_args = {'specs': specs}