Can't Form.add_error() in FormView.form_valid()

[Antoine Catton]

Let's consider this example:

class MyView(FormView):
    def form_valid(self, form):
        value = form.cleaned_data['value']
        try:
            do_action(value)
        except ValueError as e:
            form.add_error(None, e.message)
            return self.form_invalid(form)
        return super().form_valid(form)

The error message is ignored. The main reason being that the form in discarded in FormView.form_invalid(). Here's the current implementation on master:

    def form_invalid(self, form):
        """
        If the form is invalid, re-render the context data with the
        data-filled form and errors.
        """
        return self.render_to_response(self.get_context_data())

I proposed to modify this implementation to:

    def form_invalid(self, form):
        return self.render_to_response(self.get_context_data(form=form))

since Form.get_context_data() is doing kwargs.setdefault('form', self.get_form()). I did that on my view, but it does feel this is something that belongs in Django.

Also, from my little understanding of the code, this also mean that if I have a form like this:

class MyForm(Form):
    def clean_value(self):
        value = self.cleaned_data['value']
        try:
            heavy_computation(value)
        except ValueError as e:
            raise ValidationError(e.message)
        return value

The heavy_computation() is going run twice.