Backward Compatible Schema Evolution in Python

import json
from copy import deepcopy


class SchemaValidator:
    def __init__(self, schema):
        self.schema = schema

    def evolve(self, new_schema):
        """Evolve mock schema while keeping backward compatibility."""
        for field in self.schema:
            if field not in new_schema:
                # Preserve old fields in evolved schema
                new_schema[field] = deepcopy(self.schema[field])
        self.schema = new_schema
        return self.schema

    def validate(self, data):
        """Mock validation: check that all required old fields exist."""
        errors = []
        for field, rules in self.schema.items():
            if rules.get("required", False) and field not in data:
                errors.append(f"Missing required field: {field}")
        return errors if errors else "Valid"


if __name__ == "__main__":
    initial_schema = {"name": {"required": True}, "age": {"required": False}}
    validator = SchemaValidator(initial_schema)

    # Simulate schema evolution adding a new field
    evolved = validator.evolve({"name": {"required": True}, "email": {"required": True}})
    print("Evolved schema:", json.dumps(evolved, indent=2))

    # Old data still validates as compatible (ignores unknown 'age')
    print("Old data:", validator.validate({"name": "Alice"}))
    print("New data:", validator.validate({"name": "Alice", "email": "alice@example.com", "age": 30}))
    print("Missing name:", validator.validate({"email": "bob@example.com"}))