[Fixed]-Django: How to add an extra form to a formset after it has been constructed?

I do this using javascript. Since the formset renders three management fields

<input type="hidden" id="id_TOTAL_FORMS" value="1" name="TOTAL_FORMS">
<input type="hidden" id="id_INITIAL_FORMS" value="1" name="INITIAL_FORMS">.
<input type="hidden" id="id_MAX_NUM_FORMS" name="MAX_NUM_FORMS">

you can use javascript to increment the id_TOTAL_FORMS value, and just add in the extra fields. So I’d create my fieldset like this:

VehicleFormSet = modelformset_factory(StaffVehicleForm, extra = 0, max_num = None)

The tricky thing is to create the extra form fields in javascript. I usually use AJAX to fetch a new row from a custom view.

For posterity here is another way which works without JS (or alongside JS) and which does not require intimate knowledge of formset methods. Instead, you can just inspect the POST data and adjust it as if JS had done some work client-side. The following makes sure that there is always (at least) one empty form at the end of the formset:

def hsview( request):
    HS_formset = formset_factory( HSTestForm, extra=3 )
    prefix='XYZZY'
    testinpost, empty = 'key', ''  # field in the form and its default/empty value
    extra=3

# I prefer to do the short init of unbound forms first, so I invert the usual test ...   
    if request.method != 'POST':

        formset = HS_formset( prefix=prefix)
    else:
       # process POSTed forms data. 
       # pull all relevant things out of POST data, because POST itself is not mutable
        # (it doesn't matter if prefix allows in extraneous items)

        data = { k:v for k,v in request.POST.items() if k.startswith(prefix) } 

        #if there are no spare empty forms, tell it we want another form, in place of or extra to client-side JS
        #don't want to crash if unvalidated POST data is nbg so catch all ...
        try:
            n = int( data[ prefix + '-TOTAL_FORMS']) 
            test = '{}-{}-{}'.format(prefix, n-1, testinpost)
            #print(test)
            test = data.get( test, empty)
        except Exception:
            test = 'bleagh'
            # log the error if it matters enough ...
        if test != empty: 
            data[ prefix + '-TOTAL_FORMS'] = n + 1 

        # now the usual formset processing ...
        formset = HS_formset( data, prefix=prefix)
        # other_form = OtherForm( request.POST)
        if formset.is_valid(): 
            ...            

I use RegEx in my Vue.js method:

addForm: function () {
    this.count++
    let form_count = this.count
    form_count++

    let formID = 'id_form-' + this.count
    incremented_form = this.vue_form.replace(/form-\d/g, 'form-' + this.count)
    this.formList.push(incremented_form)
    this.$nextTick(() => {
        let total_forms = document.getElementsByName('form-TOTAL_FORMS').forEach
        (function (ele, idx) {
            ele.value = form_count
        })
    })
},

delForm: function () {
    if (this.count != 0) {
        this.count--
        let form_count = this.count
        form_count++

        let formID = 'id_form-' + this.count
        this.formList.pop()
        this.$nextTick(() => {
            let total_forms = document.getElementsByName('form-TOTAL_FORMS').forEach
            (function (ele, idx) {
                ele.value = form_count
            })
        })
    }
    else return
},