How can I load a combobox depending on the selection of another combobox with javascript?

For your question we have to use onchange

HTML:

 <select id="comboA" onchange="getComboA(this)">
    <option value="">Select combo</option>
    <option value="Value1">Text1</option>
    <option value="Value2">Text2</option>
    <option value="Value3">Text3</option>
    </select>



 <select id="comboB">

    </select>

javascript:

 function getComboA(selectObject) {

    // aqui dependera de donde obtienes los datos para ingresar las opciones de tu combo, puede ser una invocacion `ajax`
    var option = document.createElement("option");
    option.text = "Text";
    option.value = "myvalue";
    var select = document.getElementById("comboB");
    select.appendChild(option); 
}

I hope it helps you, Greetings.

You can do something like this, where you use the change event to consult the information you handle in your models through ajax. Every time you change region, a request will be made and the options of your commune combo will be changed. I see that you use MVC so it would be to send the info to your controller

  $("#micombo").change(function(){

    var region =$("#micombo").val(); 
        var connect, form, response, result;

        form = "region="+region; //informacion que enviaras a tu .py 
        connect = window.XMLHttpRequest ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP');
        connect.onreadystatechange = function() {
            if(connect.readyState == 4 && connect.status == 200) {                  
                var datos = JSON.parse(connect.responseText);
                //luego de la respuesta del servidor recorreras todas las comunas para llenar el segundo combo
                $.each( datos, function( key, value ) {

                    $("#miselect").append("<option value ='"+value.id+"'>"+value.descripcion+"</option>");
                });
            }
        }

        connect.open('GET','miPythonController.py',true);
        connect.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
        connect.send(form);
});