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class IncidentEdit(UpdateView):
    model=Incident
    fields = visible_field_list
    sucess_url = '/status'

class IncidentForm(ModelForm):
    class Meta:
        model = Incident
        # Define fields you want here, it is best practice not to use '__all__'
        fields = [...]

    def clean(self):
        cleaned_data = super(IncidentForm, self).clean()

        field_1 = cleaned_data.get('field_1')
        field_2 = cleaned_data.get('field_2')
        field_3 = cleaned_data.get('field_3')

        # Values may be None if the fields did not pass previous validations.
        if field_1 is not None and field_2 is not None and field_3 is not None:
            # If fields have values, perform validation:
            if not field_3 == field_1 + field_2:
                # Use None as the first parameter to make it a non-field error.
                # If you feel is related to a field, use this field's name.
                self.add_error(None, ValidationError('field_3 must be equal to the sum of field_1 and filed_2'))

        # Required only if Django version < 1.7 :
        return cleaned_data


class IncidentEdit(UpdateView):
    model = Incident
    form_class = IncidentForm
    fields = visible_field_list
    success_url = '/status'

class IncidentEdit(UpdateView):

    ...

    def form_valid(self, form):
        if form.cleaned_data['email'] in
        [i.email for i in Incident.objects.exclude(id=get_object().id)]:
            # Assume incident have email and it should be unique !!
            form.add_error('email', 'Incident with this email already exist')
            return self.form_invalid(form)
        return super(IncidentEdit, self).form_valid(form)

# models.py
class Incident(models.Model):
    numboys = models.SmallIntegerField(default=0)
    numgirls = models.SmallIntegerField(default=0)
    classttl = models.SmallIntegerField(default=0)


# views.py
def retunTestPassedResp(request):
    return HttpResponse()

class NumValidationMixin:
    def form_valid(self, form):
        data = self.request.POST
        boys = data.get('numboys')
        girls = data.get('numgirls')
        ttl = data.get('classttl')
        if boys and girls and ttl:
            if int(ttl) == int(boys) + int(girls):
                return super().form_valid(form)
            # use form.errors to add the error msg as a dictonary
            form.errors['input invalid'] = '%s + %s not equal %s'%(boys, girls, ttl)
        form.errors['input invalid'] = 'losing input with boys or other'
        return self.form_invalid(form)

class UpdateIncident(NumValidationMixin, UpdateView):
    model = Incident
    fields = ['numboys', 'numgirls', 'classttl']
    success_url = reverse_lazy('test-passed')

# templates/.../Incident_form.html
[...]
<body>
    {{form}}
    {% if form.errors %}
    <p>get error</p>
        {{form.errors}}
    {% endif %}
</body>

# tests.py
class IncidentUpdateTest(TestCase):
    def setUp(self):
        Incident.objects.create()

    def test_can_update_with_right_data(self):
        [...]

    def test_invalid_error_with_illegal_post(self):
        response = self.client.post(
            reverse('update-incident', args=(1,)),
            data={'numboys': '1', 'numgirls': '1', 'classttl': '3'}
        )
        self.assertEqual(Incident.objects.first().classttl, 0)
        # testing response page showing error msg
        self.assertContains(response, 'not equal')