prevent usage of {tags} directly.

src/documents/file_handling.py

@@ -8,6 +8,12 @@ from django.conf import settings
 from django.template.defaultfilters import slugify
 
 
+class defaultdictNoStr(defaultdict):
+
+    def __str__(self):
+        raise ValueError("Don't use {tags} directly.")
+
+
 def create_source_path_directory(source_path):
     os.makedirs(os.path.dirname(source_path), exist_ok=True)
 
@@ -90,7 +96,7 @@ def generate_filename(doc, counter=0):
 
     try:
         if settings.PAPERLESS_FILENAME_FORMAT is not None:
-            tags = defaultdict(lambda: slugify(None),
+            tags = defaultdictNoStr(lambda: slugify(None),
                                     many_to_dictionary(doc.tags))
 
             if doc.correspondent:

src/documents/tests/test_file_handling.py

@@ -267,6 +267,15 @@ class TestFileHandling(DirectoriesMixin, TestCase):
         self.assertEqual(generate_filename(document),
                          "none.pdf")
 
+    @override_settings(PAPERLESS_FILENAME_FORMAT="{tags}")
+    def test_tags_without_args(self):
+        document = Document()
+        document.mime_type = "application/pdf"
+        document.storage_type = Document.STORAGE_TYPE_UNENCRYPTED
+        document.save()
+
+        self.assertEqual(generate_filename(document), f"{document.pk:07}.pdf")
+
     @override_settings(PAPERLESS_FILENAME_FORMAT="{title} {tag_list}")
     def test_tag_list(self):
         doc = Document.objects.create(title="doc1", mime_type="application/pdf")