def add_payment_method(self, payment_method, set_default=True):
        """
        Adds an already existing payment method to this customer's account

        :param payment_method: PaymentMethod to be attached to the customer
        :type payment_method: str, PaymentMethod
        :param set_default: If true, this will be set as the default_payment_method
        :type set_default: bool
        :rtype: PaymentMethod
        """
        from .payment_methods import PaymentMethod

        stripe_customer = self.api_retrieve()
        payment_method = PaymentMethod.attach(payment_method, stripe_customer)

        if set_default:
            stripe_customer["invoice_settings"]["default_payment_method"] = (
                payment_method.id
            )
            stripe_customer.save()

            # Refresh self from the stripe customer, this should have two effects:
            # 1) sets self.default_payment_method (we rely on logic in
            # Customer._manipulate_stripe_object_hook to do this)
            # 2) updates self.invoice_settings.default_payment_methods
            self.sync_from_stripe_data(stripe_customer, api_key=self.default_api_key)
            self.refresh_from_db()

        return payment_method

        Adds an already existing payment method to this customer's account

        :param payment_method: PaymentMethod to be attached to the customer
        :type payment_method: str, PaymentMethod
        :param set_default: If true, this will be set as the default_payment_method
        :type set_default: bool
        :rtype: PaymentMethod
        
djstripe/models/core.py