Sometimes it is very useful to add the form fields validation bypassing "clean" method of form. To do this in Django we can use "add_error" method. This method can be used directly from Django views. For example:

class TestFormView(FormView):
    form_class = TestForm
    template_name = 'test/form.html'

    def form_valid(self, form):
        less_than_one = form.cleaned_data.get('less_than_one')
        if less_than_one > 1:
            form.add_error('less_than_one', forms.ValidationError('Can not be greater than one'))
            return super(TestFormView, self).form_invalid(form)
        return super(TestFormView, self).form_valid(form)

"add_error" method was added in Django since version 1.7 (including). In earlier versions of Django (<=1.6) you can use this mixin:

from django import forms
from django.forms.forms import NON_FIELD_ERRORS

class AddErrorMixin(object):
    "Backport add_error() for django <1.7"
    def add_error(self, field, msg):
        field = field or NON_FIELD_ERRORS
        if field in self._errors:
            self._errors[field] = self.error_class([msg])

class ExampleForm(AddErrorMixin, forms.Form):