Who Sets The Next State

A reviewer labels this "Strategy: the order delegates to a swappable policy object".

class Order:
    def __init__(self):     self.handler = Draft(self)
    def submit(self):       self.handler.submit()
    def pay(self):          self.handler.pay()

class Draft:
    def __init__(self, order): self.order = order
    def submit(self):          self.order.handler = AwaitingPayment(self.order)
    def pay(self):             raise Invalid("submit first")

class AwaitingPayment:
    def __init__(self, order): self.order = order
    def submit(self):          raise Invalid("already submitted")
    def pay(self):             self.order.handler = Paid(self.order)

class Paid:
    def __init__(self, order): self.order = order
    def submit(self):          raise Invalid("already paid")
    def pay(self):             raise Invalid("already paid")

Which pattern is it, and which single line settles the question?