Strategy vs state selection

class Checkout: def __init__(self): self.state = 'NEW' def pay(self): if self.state == 'NEW': ... self.state = 'PAID' elif self.state == 'PAID': raise Error('already paid') elif self.state == 'SHIPPED': raise Error('too late') # behavior depends on an internal mode with transitions The object's behavior changes as it moves through defined transitions. Which pattern best models this?