Allow None for annotations without notes in wrann

docs/changes.rst

@@ -44,6 +44,9 @@ Unreleased
 **Reject aux notes that can't be written in wrann**
   ``wrann`` now raises a ``ValueError`` if an ``aux_note`` is longer than 255 characters or contains characters outside Latin-1, instead of writing a corrupt annotation file. A longer note used to be read back as many extra annotations, and other characters were silently replaced.
 
+**Allow None for annotations without notes in wrann**
+  ``wrann`` now accepts ``None`` in ``aux_note`` for annotations without a note, as the docstring says, and writes it the same way as an empty string. It used to raise a ``TypeError``.
+
 Version 4.3.1 (Feb 2026)
 -----------------------------
 

src/wfdb/io/annotation.py

@@ -514,6 +514,9 @@ def check_field(self, field):
         # The string fields
         elif field in ["symbol", "description", "aux_note"]:
             uniq_elements = set(item)
+            # None means no note, the same as an empty string
+            if field == "aux_note":
+                uniq_elements.discard(None)
 
             for e in uniq_elements:
                 if not isinstance(e, str_types):

tests/test_annotation.py

@@ -322,3 +322,23 @@ def test_aux_note_rejected(temp_path, aux_note, match):
             write_dir=temp_path,
         )
     assert not os.path.exists(os.path.join(temp_path, "bad_aux.atr"))
+
+
+def test_aux_note_none(temp_path):
+    """None in aux_note means no note, as for an empty string."""
+    for name, aux_note in [("none", [None, "(VT", None]), ("empty", ["", "(VT", ""])]:
+        wfdb.wrann(
+            name,
+            "atr",
+            np.array([1, 5, 9]),
+            symbol=["N", "N", "V"],
+            aux_note=aux_note,
+            write_dir=temp_path,
+        )
+    ann = wfdb.rdann(os.path.join(temp_path, "none"), "atr")
+    assert ann.aux_note == ["", "(VT", ""]
+    with (
+        open(os.path.join(temp_path, "none.atr"), "rb") as f1,
+        open(os.path.join(temp_path, "empty.atr"), "rb") as f2,
+    ):
+        assert f1.read() == f2.read()