form_invalid

I need get data from a form when submit is clicked even if data are invalid, inconplete or empty.
I try to override form_invalid in CreateView but it is never called and form_valid, obviously, only if data are valid.
Why form_invalid is never called?

I have this test project with django 1.10.5 and python 3.5

models.py

class Demo(models.Model):
        demo_descr = models.CharField("Description",
            max_length=50,
        )
        demo_code = models.CharField("Code",
            max_length=5,
        )

        def __str__(self):
                return self.demo_descr
   
        class Meta:
            verbose_name = "Test"
            verbose_name_plural = "Tests"
            ordering = ["demo_descr"]

views.py

class DemoForm(forms.ModelForm):
    class Meta:
        model = models.Demo
        fields = ('demo_descr','demo_code')
  

class DemoCreateView(CreateView):
    form_class = DemoForm
    success_url = 'demo'
    template_name = 'test_form.html'
    model = models.Demo

    def form_valid(self, form):
        print('valid')
        return CreateView.form_valid(self, form)

    def form_invalid(self, form):
        print('invalid')
        return CreateView.form_invalid(self, form)

urls.py

url(r'^demo$', views.DemoCreateView.as_view(),  name='demo'),


test_form.html

<form method="post" action="">
    {% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="Save" />
</form>

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