A form that validates that the user doesn't change (some) hidden inputs

forms.py

from django import forms
from django.core import signing
from django.utils.translation import ugettext_lazy as _


class SignedForm(forms.Form):
    bad_signature_error = _("You've been a naughty boy!")
    signed_fields = None

    signature = forms.CharField(widget=forms.HiddenInput, required=True)

    def __init__(self, *args, **kwargs):
        super(SignedForm, self).__init__(*args, **kwargs)
        if self.signed_fields:
            data = {field: unicode(self.initial.get(field)) if self.initial.get(field) is not None else '' for field in self.signed_fields}
            self.fields['signature'].initial = signing.dumps(data)
        else:
            self.fields['signature'].required = False

    def clean_signature(self):
        try:
            return signing.loads(self.cleaned_data['signature'])
        except signing.BadSignature:
            raise forms.ValidationError(self.bad_signature_error)


    def clean(self):
        if self.signed_fields:
            data = {field: self.data.get(field) for field in self.signed_fields}
            signed_data = self.cleaned_data['signature']
            for key, value in data.items():
                if signed_data[key] != value:
                    raise forms.ValidationError(self.bad_signature_error)
        return self.cleaned_data