Magento 2 – Keep Product Image Aspect Ratio

Magento_Catalog/templates/product/list.phtml

<?php
 //$image = 'category_page_grid' or 'category_page_list';
 $_imagehelper = $this->helper('Magento\Catalog\Helper\Image');

 $productImage = $_imagehelper->init($_product, $image)->constrainOnly(FALSE)->keepAspectRatio(TRUE)->keepFrame(FALSE)->resize(400)->getUrl();
?>

<img src="<?php echo $productImage; ?>" />