cashonly/cashonly/core/formfields.py

from django.forms import CharField
from django.core.exceptions import ValidationError
from django.utils.translation import ugettext_lazy as _
from django.core.validators import EMPTY_VALUES


class DigitField(CharField):
    def clean(self, value):
        super(DigitField, self).clean(value)

        if value not in EMPTY_VALUES and not value.isdigit():
            raise ValidationError(_('Please enter only digits.'))

        return value