flask:how to stop python programs when jquery find an invalid input?

Im new to Flask . I try a user log-in page I validate the user input by jQuery code:

$(document).ready(function(){
//alert('begin');
$(".input.reg_butn").click(function(){
    if($("[name=username]").val()==""){
        alert("username empty");
    }else{
        if($("[name=password]").val()==""){
            alert("password empty");
        }
    }
})

})

and my python code is as follow:

@app.route('/login',methods=['GET','POST'])

def login(): .....

My question is how to stop python code running when the js find that the user submit an invalid value(such as empty username of password). I user alert in JS to notify the user , BUT the python code run as usual. How to solve it ?