Magento – override checkout controller in magento 2

I try to override module-checkout/controller/cart/CouponPost.php in my custom module.

so i created a di.xml:

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\checkout\Controller\cart" type="namespace\modulename\Controller\cart\Coupontest" />
</config>

and then Controller/Cart/CouponPost path for my CouponPost.php, but it dosen't work.

Does anyone know how can i override this CouponPost.php controller in the right way?

and my controller:

<?php
/**
 * Copyright © 2016 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace namespace\DiscountScript\Controller\Cart;

class CouponPost extends \Magento\Checkout\Controller\Cart
{
    /**
     * Sales quote repository
     *
     * @var \Magento\Quote\Api\CartRepositoryInterface
     */
    protected $quoteRepository;

    /**
     * Coupon factory
     *
     * @var \Magento\SalesRule\Model\CouponFactory
     */
    protected $couponFactory;

    /**
     * @param \Magento\Framework\App\Action\Context $context
     * @param \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig
     * @param \Magento\Checkout\Model\Session $checkoutSession
     * @param \Magento\Store\Model\StoreManagerInterface $storeManager
     * @param \Magento\Framework\Data\Form\FormKey\Validator $formKeyValidator
     * @param \Magento\Checkout\Model\Cart $cart
     * @param \Magento\SalesRule\Model\CouponFactory $couponFactory
     * @param \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
     * @codeCoverageIgnore
     */
    public function __construct(
        \Magento\Framework\App\Action\Context $context,
        \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig,
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\Data\Form\FormKey\Validator $formKeyValidator,
        \Magento\Checkout\Model\Cart $cart,
        \Magento\SalesRule\Model\CouponFactory $couponFactory,
        \Magento\Quote\Api\CartRepositoryInterface $quoteRepository
    ) {
        parent::__construct(
            $context,
            $scopeConfig,
            $checkoutSession,
            $storeManager,
            $formKeyValidator,
            $cart
        );
        $this->couponFactory = $couponFactory;
        $this->quoteRepository = $quoteRepository;
    }

    /**
     * Initialize coupon
     *
     * @return \Magento\Framework\Controller\Result\Redirect
     * @SuppressWarnings(PHPMD.CyclomaticComplexity)
     * @SuppressWarnings(PHPMD.NPathComplexity)
     */
    public function execute()
    {
        $couponCode = $this->getRequest()->getParam('remove') == 1
            ? ''
            : trim($this->getRequest()->getParam('coupon_code'));

        $cartQuote = $this->cart->getQuote();
        $oldCouponCode = $cartQuote->getCouponCode();

        $codeLength = strlen($couponCode);
        if (!$codeLength && !strlen($oldCouponCode)) {
            return $this->_goBack();
        }

        try {
            $isCodeLengthValid = $codeLength && $codeLength <= \Magento\Checkout\Helper\Cart::COUPON_CODE_MAX_LENGTH;

            $itemsCount = $cartQuote->getItemsCount();
            if ($itemsCount) {
                $cartQuote->getShippingAddress()->setCollectShippingRates(true);
                $cartQuote->setCouponCode($isCodeLengthValid ? $couponCode : '')->collectTotals();
                $this->quoteRepository->save($cartQuote);
            }

            if ($codeLength) {
                $escaper = $this->_objectManager->get('Magento\Framework\Escaper');
                if (!$itemsCount) {
                    if ($isCodeLengthValid) {
                        $coupon = $this->couponFactory->create();
                        $coupon->load($couponCode, 'code');
                        if ($coupon->getId()) {
                            $this->_checkoutSession->getQuote()->setCouponCode($couponCode)->save();
                            $this->messageManager->addSuccess(
                                __(
                                    'Youx used coupon code "%1".',
                                    $escaper->escapeHtml($couponCode)
                                )
                            );
                        } else {
                            $this->messageManager->addError(
                                __(
                                    'Thex coupon code "%1" is not valid.',
                                    $escaper->escapeHtml($couponCode)
                                )
                            );
                        }
                    } else {
                        $this->messageManager->addError(
                            __(
                                'The coupon code "%1" is not valid.',
                                $escaper->escapeHtml($couponCode)
                            )
                        );
                    }
                } else {
                    if ($isCodeLengthValid && $couponCode == $cartQuote->getCouponCode()) {
                        $this->messageManager->addSuccess(
                            __(
                                'You used coupon code "%1".',
                                $escaper->escapeHtml($couponCode)
                            )
                        );
                    } else {
                        $this->messageManager->addError(
                            __(
                                'The coupon code "%1" is not valid.',
                                $escaper->escapeHtml($couponCode)
                            )
                        );
                        $this->cart->save();
                    }
                }
            } else {
                $this->messageManager->addSuccess(__('You canceled the coupon code.'));
            }
        } catch (\Magento\Framework\Exception\LocalizedException $e) {
            $this->messageManager->addError($e->getMessage());
        } catch (\Exception $e) {
            $this->messageManager->addError(__('We cannot apply the coupon code.'));
            $this->_objectManager->get('Psr\Log\LoggerInterface')->critical($e);
        }

        return $this->_goBack();
    }
}