from odoo.addons.website_sale.controllers.main import WebsiteSale
class WebsiteSaleInherit(WebsiteSale):
@http.route(['/shop/checkout'], type='http', auth="public", website=True)
def checkout(self, **post):
# use super to call prevously defined checkout methods so we not overriding the method but extend it.
res = super(WebsiteSale, self).checkout(**post)
# code your bugs in here ;)