Ever visited the python 2 documentation on accident? Ever got linked to the python 3.2 docs by google? Install this userscript to be automatically redirected to the version 3 docs whenever you visit docs.python.org.

always_python_3_docs.user.js

// ==UserScript==
// @name         always python 3 docs
// @description  Automatically redirects from python 2 or python 3.x documentation to the latest python 3 documentation
// @version      1.3.4
// @author       Paul Pinterits
// @include      /https?://docs\.python\.org/[\d.]+/.*/
// @grant        none
// @run-at       document-start
// @updateURL    https://gist.github.com/Aran-Fey/ecb80a7d3b3a955c9841bd6bea60d515/raw/always_python_3_docs.user.js
// @downloadURL  https://gist.github.com/Aran-Fey/ecb80a7d3b3a955c9841bd6bea60d515/raw/always_python_3_docs.user.js
// ==/UserScript==

(function(){
    console.log(window.location.href);

    function regex_escape(text){
        return text.replace(/[-\/\\^$*+?.()|[\]{}]/g, '\\$&');
    }

    function replace_version(url, version){
        const regex = new RegExp('(^https?://docs\.python\.org)/(?!'+regex_escape(version)+'/)([\\d.]+)(/.*)');
        const repl = '$1/'+version+'$3';

        return url.replace(regex, repl);
    }

    function show_version_switcher_if_404(){
        // If this page has no python3 version, display a link to the python 2 version
        if (document.querySelector('.version_switcher_placeholder') !== null){
            return;
        }

        const link = document.createElement('a');
        link.href = replace_version(window.location.href, '2.7');
        link.textContent = 'visit the python 2 version of this page';

        const center = document.createElement('center');
        center.style.paddingTop = '2em';
        center.appendChild(link);
        document.body.appendChild(center);
    }

    // if the user came here from the docs, don't redirect
    const prev_url = document.referrer;
    if (prev_url.match('^https?://docs\.python\.org')){
        show_version_switcher_if_404();
        return;
    }

    const TARGET_VERSION = '3';
    const target_url = replace_version(window.location.href, TARGET_VERSION);
    // if the url didn't change, do nothing
    if (target_url == window.location.href){
        show_version_switcher_if_404();
        return;
    }

    document.location.replace(target_url);
})();