@permission_checker.require('change')
def edit(request, document_id):
    Document = get_document_model()
    DocumentForm = get_document_form(Document)

    doc = get_object_or_404(Document, id=document_id)

    if not permission_policy.user_has_permission_for_instance(request.user, 'change', doc):
        return permission_denied(request)

    if request.method == 'POST':
        original_file = doc.file
        form = DocumentForm(request.POST, request.FILES, instance=doc, user=request.user)
        if form.is_valid():
            if 'file' in form.changed_data:
                # if providing a new document file, delete the old one.
                # NB Doing this via original_file.delete() clears the file field,
                # which definitely isn't what we want...
                original_file.storage.delete(original_file.name)
            doc = form.save()

            # Reindex the document to make sure all tags are indexed
            for backend in get_search_backends():
                backend.add(doc)

            messages.success(request, _("Document '{0}' updated").format(doc.title), buttons=[
                messages.button(reverse('wagtaildocs:edit', args=(doc.id,)), _('Edit'))
            ])
            return redirect('wagtaildocs:index')
        else:
            messages.error(request, _("The document could not be saved due to errors."))
    else:
        form = DocumentForm(instance=doc, user=request.user)

    filesize = None

    # Get file size when there is a file associated with the Document object
    if doc.file:
        try:
            filesize = doc.file.size
        except __HOLE__:
            # File doesn't exist
            pass

    if not filesize:
        messages.error(
            request,
            _("The file could not be found. Please change the source or delete the document"),
            buttons=[messages.button(reverse('wagtaildocs:delete', args=(doc.id,)), _('Delete'))]
        )

    return render(request, "wagtaildocs/documents/edit.html", {
        'document': doc,
        'filesize': filesize,
        'form': form,
        'user_can_delete': permission_policy.user_has_permission_for_instance(
            request.user, 'delete', doc
        ),
    })
dataset/ETHPy150Open torchbox/wagtail/wagtail/wagtaildocs/views/documents.py/edit