solution1.py

class Payment(models.Model):
    is_started = models.BooleanField(default=True)
    is_captured = models.BooleanField(default=False)
    is_completed = models.BooleanField(default=False)

    def can_capture(self):
        return self.is_started and not self.is_captured and not self.is_completed

    def can_complete(self):
        return self.is_captured

    def contact_payment_gateway(self):
        """ does payment gateway magic """

    def notify_user(self):
        """ notifies the user that their payment is complete """

    def capture(self):
        if self.can_capture():
            self.contact_payment_gateway()
            self.is_captured = True
            self.save()
        else:
            raise ValueError()

    def complete(self):
        if self.can_complete():
            self.is_completed = True
            self.save()
            self.notify_user()
        else:
            raise ValueError()