python/docs/source/_templates/version-switcher.html

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<div id="version-button" class="dropdown">
    <button type="button" class="btn btn-secondary btn-sm navbar-btn dropdown-toggle" id="version_switcher_button" data-toggle="dropdown">
        {{ release }}
        <span class="caret"></span>
    </button>
    <div id="version_switcher" class="dropdown-menu list-group-flush py-0" aria-labelledby="version_switcher_button">
    <!-- dropdown will be populated by javascript on page load -->
    </div>
</div>

<script type="text/javascript">
// Function to construct the target URL from the JSON components
function buildURL(entry) {
    var template = "{{ switcher_template_url }}";  // supplied by jinja
    template = template.replace("{version}", entry.version);
    return template;
}

// Function to check if corresponding page path exists in other version of docs
// and, if so, go there instead of the homepage of the other docs version
function checkPageExistsAndRedirect(event) {
    const currentFilePath = "{{ pagename }}.html",
          otherDocsHomepage = event.target.getAttribute("href");
    let tryUrl = `${otherDocsHomepage}${currentFilePath}`;
    $.ajax({
        type: 'HEAD',
        url: tryUrl,
        // if the page exists, go there
        success: function() {
            location.href = tryUrl;
        }
    }).fail(function() {
        location.href = otherDocsHomepage;
    });
    return false;
}

// Function to populate the version switcher
(function () {
    // get JSON config
    $.getJSON("{{ switcher_json_url }}", function(data, textStatus, jqXHR) {
        // create the nodes first (before AJAX calls) to ensure the order is
        // correct (for now, links will go to doc version homepage)
        $.each(data, function(index, entry) {
            // if no custom name specified (e.g., "latest"), use version string
            if (!("name" in entry)) {
                entry.name = entry.version;
            }
            // construct the appropriate URL, and add it to the dropdown
            entry.url = buildURL(entry);
            const node = document.createElement("a");
            node.setAttribute("class", "list-group-item list-group-item-action py-1");
            node.setAttribute("href", `${entry.url}`);
            node.textContent = `${entry.name}`;
            node.onclick = checkPageExistsAndRedirect;
            $("#version_switcher").append(node);
        });
    });
})();
</script>