class SocWorkerForm(ModelForm):
    class Meta:
        model = SocWorker
        exclude = ('agency', 'atime')
    def __init__(self, *args, **kwargs):
        self.agency = kwargs.pop('agency')
        super(SocWorkerForm, self).__init__(*args, **kwargs)
        self.fields['department'].queryset = Department.objects.filter(agency=self.agency)
        self.fields['inn'].widget.attrs = {'maxlength' : '12', 'minlength' : '10'}
        self.fields['snils'].widget.attrs = {'maxlength' : '11'}

class SocWorker(SocWorker_detailed_inf):
    class Meta:
        verbose_name = 'Работник'
        verbose_name_plural = 'Работники'
    agency = models.ForeignKey(Agency) 
    department = models.ForeignKey(Department, verbose_name='Отделение', null=True, blank=True)
    surname = models.CharField('Фамилия', max_length=256)
# Абстрактный класс, использует доп. поля (с именами атрибутов этого класса) для SocWorker
class SocWorker_detailed_inf(models.Model):
    inn = models.IntegerField('ИНН', max_length=10, blank=True)
    snils = models.IntegerField('СНИЛС', max_length=11, blank=True)

    
from django.utils.translation import ugettext_lazy as _   
def clean_inn(self):
    inn = self.cleaned_data.get['inn']
    if len(inn) < 10:
        raise forms.ValidationError(_("Wrong inn length"))
    return inn    
def clean_snils(self):
    snils = self.cleaned_data['snils']
    if len(snils) < 10:
        raise forms.ValidationError(_("Wrong snils length"))
    return snils

    def clean_inn(self):
        data = str(self.cleaned_data['inn'])
        if len(data)<10:
            raise forms.ValidationError(u"Длина ИНН должна быть больше")
        return data
        data = int(data)
        self._errors['inn'] = self.error_class([u"Длина ИНН должна быть больше"])
    def clean_snils(self):
        snilsd = str(self.cleaned_data['snils'])
        if len(snilsd)<11:
            raise forms.ValidationError(u"Длина СНИЛС должна быть больше")
        return snilsd
        snilsd = int(snilsd)
        self._errors['snils'] = self.error_class([u"Длина СНИЛС должна быть больше"])

   def clean_snils(self):
        snils = str(self.cleaned_data['snils'])
        if  len(snils)>=1 and len(snils)<11:
            raise forms.ValidationError(u"Длина СНИЛС должна быть больше")
        return snils

 = forms.CharField(required=False)

class SocWorkerForm(ModelForm):
    class Meta:
        model = SocWorker

class SocWorker_detailed_inf(models.Model):
    inn = models.IntegerField('ИНН', max_length=10, blank=True, null=True)
class SocWorker(SocWorker_detailed_inf):
....

    def clean_inn(self):
        inn = self.cleaned_data['inn']
        if inn is not None:
            inn=str(inn)            
            if len(inn)>=1 and len(inn)<10:
                raise forms.ValidationError(_(u"Длина ИНН должна быть больше"))
                inn=int(inn)
        return inn

class SocWorkerForm(ModelForm):
    class Meta:
        model = SocWorker
    def __init__(self, *args, **kwargs):
        super(SocWorkerForm, self).__init__(*args, **kwargs)
        self.fields['inn'].error_messages = {'inn': 'Test'}

    def clean_inn(self):
        inn = self.cleaned_data['inn']
        if inn is not None:
            inn=str(inn)            
            if len(inn)>=1 and len(inn)<10:
                raise forms.ValidationError(_(u"Длина ИНН должна быть больше"))
                inn=int(inn)
        return inn

raise forms.ValidationError(_(u"Длина ИНН должна быть больше"))

raise forms.ValidationError('inn')