from schema import And, Schema, Use


@dataclass
class Allocate(Command):

    _schema = Schema({  #(1)
        'orderid': str,
        'sku': str,
        'qty': And(Use(int), lambda n: n > 0)
     }, ignore_extra_keys=True)

    orderid: str
    sku: str
    qty: int

    @classmethod
    def from_json(cls, data):  #(2)
        data = json.loads(data)
        return cls(**_schema.validate(data))

def command(name, **fields):  #(1)
    schema = Schema(And(Use(json.loads), fields), ignore_extra_keys=True)
    cls = make_dataclass(name, fields.keys())  #(2)
    cls.from_json = lambda s: cls(**schema.validate(s))  #(3)
    return cls

def greater_than_zero(x):
    return x > 0

quantity = And(Use(int), greater_than_zero)  #(4)

Allocate = command(  #(5)
    'Allocate',
    orderid=int,
    sku=str,
    qty=quantity
)

AddStock = command(
    'AddStock',
    sku=str,
    qty=quantity

class MessageBus:

    def handle_message(self, name: str, body: str):
        try:
            message_type = next(mt for mt in EVENT_HANDLERS if mt.__name__ == name)
            message = message_type.from_json(body)
            self.handle([message])
        except StopIteration:
            raise KeyError(f"Unknown message name {name}")
        except ValidationError as e:
            logging.error(
                f'invalid message of type {name}\n'
                f'{body}\n'
                f'{e}'
            )
            raise e

@app.route("/change_quantity", methods=['POST'])
def change_batch_quantity():
    try:
        bus.handle_message('ChangeBatchQuantity', request.body)
    except ValidationError as e:
        return bad_request(e)
    except exceptions.InvalidSku as e:
        return jsonify({'message': str(e)}), 400

def bad_request(e: ValidationError):
    return e.code, 400

def handle_change_batch_quantity(m, bus: messagebus.MessageBus):
    try:
        bus.handle_message('ChangeBatchQuantity', m)
    except ValidationError:
        print('Skipping invalid message')
    except exceptions.InvalidSku as e:
        print(f'Unable to change stock for missing sku {e}')

{
  "orderid": "superman",
  "sku": "zygote",
  "qty": -1
}

"""
This module contains preconditions that we apply to our handlers.
"""

class MessageUnprocessable(Exception):  #(1)

    def __init__(self, message):
        self.message = message

class ProductNotFound(MessageUnprocessable):  #(2)
    """"
    This exception is raised when we try to perform an action on a product
    that doesn't exist in our database.
    """"

    def __init__(self, message):
        super().__init__(message)
        self.sku = message.sku

def product_exists(event, uow):  #(3)
    product = uow.products.get(event.sku)
    if product is None:
        raise ProductNotFound(event)

# services.py

from allocation import ensure

def allocate(event, uow):
    line = model.OrderLine(event.orderid, event.sku, event.qty)
    with uow:
        ensure.product_exists(event, uow)

        product = uow.products.get(line.sku)
        product.allocate(line)
        uow.commit()

class SkipMessage (Exception):
    """"
    This exception is raised when a message can't be processed, but there's no
    incorrect behavior. For example, we might receive the same message multiple
    times, or we might receive a message that is now out of date.
    """"

    def __init__(self, reason):
        self.reason = reason

def batch_is_new(self, event, uow):
    batch = uow.batches.get(event.batchid)
    if batch is not None:
        raise SkipMessage(f"Batch with id {event.batchid} already exists")

class MessageBus:

    def handle_message(self, message):
        try:
            ...
        except SkipMessage as e:
            logging.warn(f"Skipping message {message.id} because {e.reason}")