diff --git a/src/vak/prep/frame_classification/source_files.py b/src/vak/prep/frame_classification/source_files.py
index 6b9fd1333..026f0dade 100644
--- a/src/vak/prep/frame_classification/source_files.py
+++ b/src/vak/prep/frame_classification/source_files.py
@@ -1,3 +1,9 @@
+"""Get source files for a dataset, or make them.
+
+Gets either audio or spectrogram files from ``data dir``,
+possibly paired with annotation files."""
+from __future__ import annotations
+
 import logging
 import pathlib