fix(python): guard against None before .decode() on split_names metadata key (#3229)

`.get(b"split_names", None).decode()` was called unconditionally in both
Permutations.__init__ and Permutation.from_tables(), crashing with
AttributeError when schema metadata existed but lacked the split_names
key. Guard the decode behind a None check and add regression tests.

python/python/lancedb/permutation.py

@@ -284,9 +284,8 @@ class Permutations:
         self.permutation_table = permutation_table
 
         if permutation_table.schema.metadata is not None:
-            split_names = permutation_table.schema.metadata.get(
-                b"split_names", None
-            ).decode("utf-8")
+            raw = permutation_table.schema.metadata.get(b"split_names")
+            split_names = raw.decode("utf-8") if raw is not None else None
             if split_names is not None:
                 self.split_names = json.loads(split_names)
                 self.split_dict = {
@@ -460,9 +459,8 @@ class Permutation:
                         f"Cannot create a permutation on split `{split}`"
                         " because no split names are defined in the permutation table"
                     )
-                split_names = permutation_table.schema.metadata.get(
-                    b"split_names", None
-                ).decode("utf-8")
+                raw = permutation_table.schema.metadata.get(b"split_names")
+                split_names = raw.decode("utf-8") if raw is not None else None
                 if split_names is None:
                     raise ValueError(
                         f"Cannot create a permutation on split `{split}`"

python/python/tests/test_permutation.py

@@ -522,6 +522,50 @@ def test_no_split_names(some_table: Table):
     assert permutations[1].num_rows == 500
 
 
+def test_permutations_metadata_without_split_names_key(mem_db: DBConnection):
+    """Regression: schema metadata present but missing split_names key must not crash.
+
+    Previously, `.get(b"split_names", None).decode()` was called unconditionally,
+    so any permutation table whose metadata dict had other keys but no split_names
+    raised AttributeError: 'NoneType' has no attribute 'decode'.
+    """
+    base = mem_db.create_table("base_nosplit", pa.table({"x": range(10)}))
+
+    # Build a permutation-like table that carries some metadata but NOT split_names.
+    raw = pa.table(
+        {
+            "row_id": pa.array(range(10), type=pa.uint64()),
+            "split_id": pa.array([0] * 10, type=pa.uint32()),
+        }
+    ).replace_schema_metadata({b"other_key": b"other_value"})
+    perm_tbl = mem_db.create_table("perm_nosplit", raw)
+
+    permutations = Permutations(base, perm_tbl)
+    assert permutations.split_names == []
+    assert permutations.split_dict == {}
+
+
+def test_from_tables_string_split_missing_names_key(mem_db: DBConnection):
+    """Regression: from_tables() with a string split must raise ValueError, not
+    AttributeError.
+
+    Previously, `.get(b"split_names", None).decode()` crashed with AttributeError
+    when the metadata dict existed but had no split_names key.
+    """
+    base = mem_db.create_table("base_strsplit", pa.table({"x": range(10)}))
+
+    raw = pa.table(
+        {
+            "row_id": pa.array(range(10), type=pa.uint64()),
+            "split_id": pa.array([0] * 10, type=pa.uint32()),
+        }
+    ).replace_schema_metadata({b"other_key": b"other_value"})
+    perm_tbl = mem_db.create_table("perm_strsplit", raw)
+
+    with pytest.raises(ValueError, match="no split names are defined"):
+        Permutation.from_tables(base, perm_tbl, split="train")
+
+
 @pytest.fixture
 def some_perm_table(some_table: Table) -> Table:
     return (