how to to Serialize Custom Data Types with Marshmallow in Flask REST API
I've been researching this but I'm collaborating on a project where I'm sure I'm missing something obvious here, but I'm working on a Flask REST API and trying to serialize a custom Python object using Marshmallow. I have a class `User` that has a nested `Address` object, and I'm trying to serialize it using a nested schema. However, I'm working with a `TypeError: Object of type Address is not JSON serializable` when I try to return the serialized data. Here's a simplified version of my code: ```python from flask import Flask, jsonify from flask_marshmallow import Marshmallow app = Flask(__name__) ma = Marshmallow(app) class Address: def __init__(self, street, city): self.street = street self.city = city class User: def __init__(self, name, address): self.name = name self.address = address class AddressSchema(ma.Schema): class Meta: fields = ('street', 'city') class UserSchema(ma.Schema): address = ma.Nested(AddressSchema) class Meta: fields = ('name', 'address') @app.route('/user', methods=['GET']) def get_user(): address = Address('123 Main St', 'Anytown') user = User('John Doe', address) user_schema = UserSchema() return jsonify(user_schema.dump(user)) if __name__ == '__main__': app.run() ``` I’ve verified that the `Address` class is being instantiated correctly, but since it is just a plain Python object and not a dataclass or a dictionary, Marshmallow doesn’t know how to handle it by default. I've read through the Marshmallow documentation but I'm unsure how to modify my existing code to ensure that the `Address` object gets serialized properly. I’ve tried adding a `@ma.post_load` method to convert `Address` to a dictionary, but that hasn’t resolved the serialization scenario. Any suggestions on how to properly serialize this nested object structure with Marshmallow? I'm working on a service that needs to handle this. I'd really appreciate any guidance on this. This issue appeared after updating to Python 3.10. Am I approaching this the right way?