django admin inlines readonly on change

class Inline(admin.TabularInline):
    model = MyModel
    extra = 0


class InlineReadOnly(Inline):
    can_delete = False

    def has_add_permission(self, request):
        return False

    def get_readonly_fields(self, request, obj=None):
        result = list(set(
                [field.name for field in self.opts.local_fields] +
                [field.name for field in self.opts.local_many_to_many]
            ))
        result.remove('id')
        return result


class MyAdmin(admin.ModelAdmin):
     def add_view(self, request, form_url='', extra_context=None):
        self.inlines = [Inline, ]
        return super(MyAdmin, self).add_view(request, form_url, extra_context)

    def change_view(self, request, object_id, form_url='', extra_context=None):
        self.inlines = [AttachmentInlineReadOnly, ]
        return super(InlineReadOnly, self).change_view(request, object_id, form_url, extra_context)