CBV: PreviewMixin

from django import forms
from formtools.utils import form_hmac
import copy


class PreviewMixin(object):
    '''
    Mixin for a 2-stage form

    For usage with the default django class-based views (CreateView, UpdateView, FormView)

    Procedure:
    First display a form in a template.
    When submitted, execute process_preview, then redisplay the form.
    If submitted again and the data matches the previous submit, run done.
    Else run process_preview again and redisplay the form once again.
    '''
    stage = 1
    preview_template_name = None

    def get_template_names(self):
        '''
        Depending on the stage, return the appropriate template
        Use the default template of the Django View for the first stage
        If a different template is wanted for the second stage, preview_template_name can be set.
        Otherwise the default template of the Django View is also used for the second stage.
        '''
        if self.stage == 2 and self.preview_template_name is not None:
            return [self.preview_template_name]
        return super(PreviewMixin, self).get_template_names()

    def get_form(self, form_class=None):
        '''
        Add a hash value to each form.
        If stage==1 (first form or previous invalid data), pass 0 as hash value
        '''
        form = super(PreviewMixin, self).get_form(form_class)
        if self.stage == 2:
            form.fields['hash'] = forms.CharField(initial='0', widget=forms.HiddenInput)
        return form
    def form_valid(self, form):
        '''
        Check if form was submitted the first or second time.
        If it is the second time and the hash-value matches, run done()
        Else run process_preview and return the second form.
        Should not be overwritten.
        '''
        if self.security_hash(form) == self.request.POST.get('hash', '0'):
            return self.done(form)
        self.process_preview(form)
        self.stage = 2
        self.request.POST['hash'] = self.security_hash(form)
        form = self.get_form(self.get_form_class())
        return self.render_to_response(self.get_context_data(form=form))

    def security_hash(self, form):
        '''
        Function to return the hash value of the form
        '''
        hash_form = copy.copy(form)
        hash_form.fields.pop('hash', '')
        return form_hmac(hash_form)

    def process_preview(self, form):
        '''
        Function which is run after the form was valid for the first time.
        Does nothing by default.
        '''
        pass

    def done(self, form):
        '''
        Function which is run after the form was submitted the second time if it is valid and the hash values match.
        By default does the same as the corresponding form_valid function of the class-based view it inherits from.
        '''
        return super(PreviewMixin, self).form_valid()