Django (extreme case): How to raise form invalid inside form_valid method of a FormView (CreateView/UpdateView) and add an error message to not field errors

views.py

from django.forms.util import ErrorList
from django import forms

class ContractUpdateView(UpdateView):
    model = Contract
    template_name = 'contract/contract_form.html'
    form_class = ContractForm

    def form_valid(self, form):
        if self.request.POST.get('finish'):
            if (
                not form.cleaned_data['number'] or
                not self.object.images.count()
            ):
                form._errors[forms.forms.NON_FIELD_ERRORS] = ErrorList([
                    u'You have to add images and a contract number'
                ])
                return self.form_invalid(form)
            self.object.is_submitted = True
        self.object = form.save()
        return HttpResponseRedirect(self.get_success_url())

add_error() is now the best way to do this. https://docs.djangoproject.com/en/2.0/ref/forms/api/#django.forms.Form.add_error

Massively helpful, thank you! Couldn't get the validation to work in the ModelForm clean() method but this did the trick.