send array from ajax to Flask not working

I am trying to send an array to flask through ajax post call. But somehow it is not working.

Javascript

<script type="text/javascript">
        function fillChart()
        {

            var nids = document.getElementById("nodes-select").value;
            var cfilter = document.getElementById("filter-select").value;
            var chkd = document.getElementById("further-select");
            var cids = [];
            for (var i=0;i<chkd.length;i++)
            {

                if(chkd[i].selected)
                {
                    cids.push(chkd[i].value);
                }
            }

            alert(cids);
            $.post("/pie",{"node_id":nids,"col_select":cfilter,"col_filter":cids},function(data,status)
            {
                var tmp = data;            
                console.log(data.otstr);                     

            });           
        }
 </script>

Server code

@app.route('/pie',methods=['POST'])
def pie():
    tmp1 = request.form.get('node_id')  
    tmp2 = request.form.get('col_select')   
    tmp3 = request.form.get('col_filter[]') 
    return jsonify(otstr=[tmp1,tmp2,tmp3])

Here tmp1 and tmp2 are just strings and tmp3 is an array of strings.console.log(data.otstr) is printing correct values of tmp1,tmp2 but when it comes to tmp3 since it is an array, it is printing the first element only.