order_state_pattern.py

# Example of state pattern by code EV
# https://youtu.be/8rynRTOr4mE

class State:
    def receive_payment(self):
        raise NotImplementedError()
    
    def ship(self):
        raise NotImplementedError()
    
    def mark_delivered(self):
        raise NotImplementedError()


class Order:  
    """Context class 'state of states' """
    def __init__(self) -> None:
        self.unpaid_state = UnpaidState(self)
        self.paid_state = PaidState(self)
        self.shipped_state = ShippedState(self)
        self.delivered_state = DeliveredState(self)

        self.state = self.unpaid_state

    def set_state(self, state: State):
        self.state = state

    def ship(self):
        self.state.ship()

    def receive_payment(self):
        self.state.receive_payment()

    def mark_delivered(self):
        self.state.mark_delivered()

    
class UnpaidState(State):
    def __init__(self, context: Order):
        self.context = context

    def __str__(self):
        return __class__.__name__
  
    def receive_payment(self):
        self.context.set_state(self.context.paid_state)
        print("Your payment was accepted")

    def ship(self):
        print("Can't ship unpaid orders")

    def mark_delivered(self):
        print("Can't deliver unpaid orders")


class PaidState(State):
    def __init__(self, context: Order):
        self.context = context

    def __str__(self):
        return __class__.__name__
     
    def receive_payment(self): 
        print("Order has already been paid")

    def ship(self):
        self.context.set_state(self.context.shipped_state)
        print("The order has been shipped")

    def mark_delivered(self):
        print("Only shipped orders can be marked delivered")
   

class ShippedState(State): ###
    def __init__(self, context: Order):
        self.context = context

    def __str__(self):
        return __class__.__name__
     
    def receive_payment(self):
        print("Order has already been paid")

    def ship(self):  
        print("The order has been shipped")

    def mark_delivered(self):
        self.context.set_state(self.context.delivered_state)
        print("Your order has been delivered")
   

   
class DeliveredState(State):
    def __init__(self, context: Order):
        self.context = context

    def __str__(self):
        return __class__.__name__
    
    def receive_payment(self): 
        print("Order has already been paid")

    def ship(self):
        self.context.set_state(self.context.shipped_state)
        print("The order has already been delivered, checkthe front door")

    def mark_delivered(self):
        print("The order has been delivered! Check the front door!")
   

if __name__ == "__main__":
    order = Order()
    print(order.state)
    order.ship()                # should generate error message
    order.receive_payment()  
    print(order.state)
    order.ship()
    order.receive_payment()     # should generate error message
    print(order.state)
    order.mark_delivered()
    print(order.state)