If you are looking for automating this process, you can create a custom widget for the field. In the widget class you will define a render method in such a way that it will also return the event binding code.

class CustomWidget(forms.TextInput):

    class Media:
        #js here
        js = ( js/my_js.js ,)

    def render(self, name, value, attrs = None)
         output = super(CustomWidget, self).render(name, value, attrs)
         #do what you want to do here
         output += ...
         return output

What I do for that is :

class MyForm(forms.Form):
    stuff = forms.ChoiceField(
        [( a , A ),( b , B )],
        widget = forms.Select(attrs = {
             onclick  : "alert( foo ! );",
            }
        )

I would recommend looking at using a JavaScript library such as jQuery. Here is the link to binding the click event in jQuery. Since Django names all of the input fields you can connect my_field_name in your Django form to the click event like so:

$("form input[name= my_field_name ]").click(function () { 
    // Handle the click event here
});

Using YUI, you can do that like this:

YAHOO.util.Event.addListener(id_myField, "click", myClickEventHandler, myOptionalData);

See also YUI 2: Event Utility

I prefer using a JavaScript library, as this keeps browser-side JS code nicely separated from server-side Django.

The Ghislain Leveque answer works to me, but in django 1.9 I did it like this:

class myForm(forms.ModelForm):
    class Meta:
        model = USER
        fields = ["password", "idCity"]
        widgets = {
            "password": forms.PasswordInput(),
            "idCity": forms.Select(attrs={ onchange : "alert( foo ! );"})
        }

I took JQuery approach using Ghislain Leveque s answer. This way I was able to trigger the event handler on page load.

class MyForm(forms.Form):
    stuff = forms.ChoiceField(
        [( a , A ),( b , B )],
        widget = forms.Select(attrs = {
             widget_to_bind  : "True",
            }
        )

In my javascript:

function myFunction() {
    var source = $(this); // Widget element
}

$( [widget_to_bind=True] ).change(myFunction);
$( [widget_to_bind=True] ).trigger("change");

Variation on a theme:

class CategoriesForm(forms.Form):
stuff = forms.MultipleChoiceField(
    choices=([(pt.id, pt.name) for pt in PaymentType.objects.order_by( name )]),
    widget=forms.SelectMultiple(attrs={ size :  20 ,  class :  mc_payment_type })
    )

JQuery:

<script language="javascript">
    $( .mc_payment_type ).change(function () {
        console.log( payment type selected:   + $(this).find( option:selected ).val());
</script>