code: def add_coupon(self, coupon, idempotency_key=None):
  		
  		if isinstance(coupon, StripeModel):
  			coupon = coupon.id
  
  		stripe_customer = self.api_retrieve()
  		stripe_customer["coupon"] = coupon
  		stripe_customer.save(idempotency_key=idempotency_key)
  		return self.__class__.sync_from_stripe_data(stripe_customer)
docs: Add a coupon to a Customer.
  
  		The coupon can be a Coupon object, or a valid Stripe Coupon ID.