Is it possible to define a nested schema with mutually exclusive fields in marshmallow?

Question

I am using marshmallow to validate json data that I am receiving in a flask restful api. In the post request however there is a mutually exclusive field.
For example : {"predict": {"id": "5hgy667y4h7f"}} or {"predict": {"text": "This is a sample sentence"}}
But NOT both id and text should be sent together. Moreover different methods are called based on weather id or text is received.

Q) How do I construct a schema in marshmallow that allows me to validate the above?

Sample code I have for either one of the fields is below -

from flask import Flask, request
from flask_restful import Resource, Api, abort
from marshmallow import Schema, fields, ValidationError
app = Flask(__name__)
api = Api(app)

class Mutex1(Schema):
    text = fields.Str(required=True)
    class Meta:
        strict = True

class Mutex2(Schema):
    id_ = fields.Str(required=True)
    class Meta:
        strict = True

class MySchema(Schema):
    predict = fields.Nested(Mutex1)
    class Meta:
        strict = True

class Test(Resource):
    def post(self):
        input_req = request.get_json(force=True)
        try:
            result = MySchema().load(input_req)
        except ValidationError:
            return {'message': 'Validation Error'}, 500
        else:
            return {'message': 'Successful validation'}, 200

api.add_resource(Test, '/test')
app.run(host='0.0.0.0', port=5000, debug=True)

This code accepts only text, and text with id_, however it rejects only id_. Any idea how to make it accept id_ and reject both text and id_ when passed together ?

Answer 1

Create a Mutex schema with both text and id_ and add a schema-level validation to fail if both are provided.

class Mutex(Schema):

    @validates_schema
    def validate_numbers(self, data):
        if (
               ('text' in data and 'id_' in data) or
               ('text' not in data and 'id_' not in data)
           ):
            raise ValidationError('Only one of text and _id is allowed')

    text = fields.Str()
    id_ = fields.Str()
    class Meta:
        strict = True

Side notes:

• Input validation error should not return a 500 (server error) but a 422.
• I'm not familiar with flask-restful, but it looks like you could save yourself some boilerplate by using webargs to parse the resource inputs.