How to Validate Request Body JSON Against a Schema in Python

import json


def validate_against_schema(data, schema, path=""):
    errors = []

    if not isinstance(data, dict):
        errors.append(f"{path}: expected object, got {type(data).__name__}")
        return errors

    for field, rules in schema.items():
        field_path = f"{path}.{field}" if path else field

        if field not in data:
            if rules.get("required", False):
                errors.append(f"{field_path}: missing required field")
            continue

        value = data[field]
        expected_type = rules.get("type")

        if expected_type:
            if not isinstance(value, expected_type):
                errors.append(
                    f"{field_path}: expected {expected_type.__name__}, got {type(value).__name__}"
                )
                continue

        if expected_type is str and "min_length" in rules:
            if len(value) < rules["min_length"]:
                errors.append(
                    f"{field_path}: length {len(value)} < {rules['min_length']}"
                )

        if "allowed" in rules:
            if value not in rules["allowed"]:
                errors.append(f"{field_path}: value must be one of {rules['allowed']}")

        if expected_type is dict and "properties" in rules:
            nested_errors = validate_against_schema(
                value, rules["properties"], field_path
            )
            errors.extend(nested_errors)

    return errors


if __name__ == "__main__":
    schema = {
        "username": {"type": str, "required": True, "min_length": 3},
        "email": {"type": str, "required": True},
        "role": {"type": str, "required": True, "allowed": ["admin", "user", "guest"]},
        "age": {"type": int, "required": False},
        "address": {
            "type": dict,
            "required": False,
            "properties": {
                "city": {"type": str, "required": True},
                "zip": {"type": str, "required": True}
            }
        }
    }

    request_body = {
        "username": "alice",
        "email": "alice@example.com",
        "role": "admin"
    }

    result = validate_against_schema(request_body, schema)
    print("Valid" if not result else "Errors: " + json.dumps(result))