fix: reject empty column alterations and unknown Python keys (#4417)

Fixes #4390

Lance accepts column alterations with every change field unset, so a
path-only Python call could commit a new local version without changing
the schema. The Python binding also silently discarded unknown keys such
as `nulable`.

Validate every alteration in the shared Rust `Table` API before
dispatching to either backend, and reject unknown dictionary keys in the
Python binding while preserving the legacy `name` alias. Invalid batches
leave local schema and version unchanged and send no remote request.

Validation:

- Reproduced both reported calls on the original extension: versions
advanced from 1 to 2 to 3 with an unchanged schema; all 13 new
invalid-input Python cases failed before the fix.
- Rebuilt the extension with `uv run --extra tests --extra dev maturin
develop --extras tests,dev`; all 18 focused Python cases pass across
sync/async and local/mocked remote tables.
- `cargo test --quiet --features remote -p lancedb --lib alter_column`:
all eight tests pass, including both new regressions.
- `cargo clippy --quiet --features remote --tests --examples` passes
across the workspace.
- `cargo fmt --all`, `ruff check .`, and `ruff format .` pass.
- `typos` 1.26.0 passes across the repository; intentional misspellings
in regression fixtures use the existing line-specific spelling
exemptions.

<!-- lance-gatekeeper-fix:v1 agent=36a547cd11108898893456042e38628b
generation=1 -->

---------

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/lancedb/table.py

@@ -2617,6 +2617,10 @@ class Table(ABC):
                 to nullable. Currently, you cannot change a nullable column to
                 non-nullable.
 
+            Each alteration must specify at least one of "rename", "data_type",
+            or "nullable". The legacy key "name" is also accepted for renaming.
+            Unknown keys raise ValueError before any alterations are applied.
+
         Returns
         -------
         AlterColumnsResult
@@ -6874,6 +6878,10 @@ class AsyncTable:
                 to nullable. Currently, you cannot change a nullable column to
                 non-nullable.
 
+            Each alteration must specify at least one of "rename", "data_type",
+            or "nullable". The legacy key "name" is also accepted for renaming.
+            Unknown keys raise ValueError before any alterations are applied.
+
         Returns
         -------
         AlterColumnsResult

python/python/tests/test_remote_db.py

@@ -119,6 +119,77 @@ async def test_async_remote_db():
         assert table_names == []
 
 
+@pytest.mark.parametrize(
+    "alteration, match",
+    [
+        ({"path": "id"}, "One of rename, nullable or data_type"),
+        (
+            {"path": "id", "nulable": False},  # spellchecker:disable-line
+            "Unknown column alteration key 'nulable'",  # spellchecker:disable-line
+        ),
+        (
+            {
+                "path": "id",
+                "rename": "new_id",
+                "nulable": False,  # spellchecker:disable-line
+            },
+            "Unknown column alteration key 'nulable'",  # spellchecker:disable-line
+        ),
+    ],
+)
+def test_remote_alter_columns_rejects_invalid_before_request(alteration, match):
+    requests = []
+
+    def handler(request):
+        requests.append(request.path)
+        request.send_response(200)
+        request.send_header("Content-Type", "application/json")
+        request.end_headers()
+        request.wfile.write(b'{"version": 1, "schema": {"fields": []}}')
+
+    with mock_lancedb_connection(handler) as db:
+        table = db.open_table("test")
+        requests.clear()
+
+        with pytest.raises(ValueError, match=match):
+            table.alter_columns(alteration)
+
+        assert requests == []
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "alteration, match",
+    [
+        ({"path": "id"}, "One of rename, nullable or data_type"),
+        (
+            {"path": "id", "nulable": False},  # spellchecker:disable-line
+            "Unknown column alteration key 'nulable'",  # spellchecker:disable-line
+        ),
+    ],
+)
+async def test_async_remote_alter_columns_rejects_invalid_before_request(
+    alteration, match
+):
+    requests = []
+
+    def handler(request):
+        requests.append(request.path)
+        request.send_response(200)
+        request.send_header("Content-Type", "application/json")
+        request.end_headers()
+        request.wfile.write(b'{"version": 1, "schema": {"fields": []}}')
+
+    async with mock_lancedb_connection_async(handler) as db:
+        table = await db.open_table("test")
+        requests.clear()
+
+        with pytest.raises(ValueError, match=match):
+            await table.alter_columns(alteration)
+
+        assert requests == []
+
+
 @pytest.mark.asyncio
 async def test_async_checkout():
     def handler(request):

python/python/tests/test_table.py

@@ -4170,14 +4170,74 @@ async def test_add_columns_with_schema(mem_db_async: AsyncConnection):
     )
 
 
-def test_alter_columns(mem_db: DBConnection):
+@pytest.mark.parametrize("rename_key", ["rename", "name"])
+def test_alter_columns(mem_db: DBConnection, rename_key):
     data = pa.table({"id": [0, 1]})
     table = mem_db.create_table("my_table", data=data)
-    alter_columns_res = table.alter_columns({"path": "id", "rename": "new_id"})
+    alter_columns_res = table.alter_columns({"path": "id", rename_key: "new_id"})
     assert alter_columns_res.version == 2
     assert table.to_arrow().column_names == ["new_id"]
 
 
+INVALID_COLUMN_ALTERATIONS = [
+    (({"path": "id"},), "One of rename, nullable or data_type"),
+    (
+        ({"path": "id", "nulable": False},),  # spellchecker:disable-line
+        "Unknown column alteration key 'nulable'",  # spellchecker:disable-line
+    ),
+    (
+        (
+            {
+                "path": "id",
+                "rename": "new_id",
+                "nulable": False,  # spellchecker:disable-line
+            },
+        ),
+        "Unknown column alteration key 'nulable'",  # spellchecker:disable-line
+    ),
+    (
+        ({"path": "id", "rename": "new_id"}, {"path": "id"}),
+        "One of rename, nullable or data_type",
+    ),
+]
+
+
+def test_alter_columns_nullable_false(mem_db: DBConnection):
+    table = mem_db.create_table("my_table", data=pa.table({"id": [0, 1]}))
+    result = table.alter_columns({"path": "id", "nullable": False})
+    assert result.version == 2
+    assert not table.schema.field("id").nullable
+
+
+@pytest.mark.parametrize("alterations, match", INVALID_COLUMN_ALTERATIONS)
+def test_alter_columns_rejects_invalid(mem_db: DBConnection, alterations, match):
+    table = mem_db.create_table("my_table", data=pa.table({"id": [0, 1]}))
+    initial_version = table.version
+    initial_schema = table.schema
+
+    with pytest.raises(ValueError, match=match):
+        table.alter_columns(*alterations)
+
+    assert table.version == initial_version
+    assert table.schema == initial_schema
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("alterations, match", INVALID_COLUMN_ALTERATIONS)
+async def test_alter_columns_rejects_invalid_async(
+    mem_db_async: AsyncConnection, alterations, match
+):
+    table = await mem_db_async.create_table("my_table", data=pa.table({"id": [0, 1]}))
+    initial_version = await table.version()
+    initial_schema = await table.schema()
+
+    with pytest.raises(ValueError, match=match):
+        await table.alter_columns(*alterations)
+
+    assert await table.version() == initial_version
+    assert await table.schema() == initial_schema
+
+
 def test_update_field_metadata(mem_db: DBConnection):
     data = pa.table({"id": [0, 1], "category": ["a", "b"]})
     table = mem_db.create_table("my_table", data=data)
@@ -4203,10 +4263,11 @@ def test_update_field_metadata(mem_db: DBConnection):
 
 
 @pytest.mark.asyncio
-async def test_alter_columns_async(mem_db_async: AsyncConnection):
+@pytest.mark.parametrize("rename_key", ["rename", "name"])
+async def test_alter_columns_async(mem_db_async: AsyncConnection, rename_key):
     data = pa.table({"id": [0, 1]})
     table = await mem_db_async.create_table("my_table", data=data)
-    alter_columns_res = await table.alter_columns({"path": "id", "rename": "new_id"})
+    alter_columns_res = await table.alter_columns({"path": "id", rename_key: "new_id"})
     assert alter_columns_res.version == 2
     assert (await table.to_arrow()).column_names == ["new_id"]
     alter_columns_res = await table.alter_columns(

python/src/table.rs

@@ -1897,6 +1897,17 @@ impl Table {
         let alterations = alterations
             .iter()
             .map(|alteration| {
+                for key in alteration.keys().iter() {
+                    let key = key.extract::<String>()?;
+                    if !matches!(
+                        key.as_str(),
+                        "path" | "rename" | "name" | "nullable" | "data_type"
+                    ) {
+                        return Err(PyValueError::new_err(format!(
+                            "Unknown column alteration key '{key}'"
+                        )));
+                    }
+                }
                 let path = alteration
                     .get_item("path")?
                     .ok_or_else(|| PyValueError::new_err("Missing path"))?

rust/lancedb/src/remote/table.rs

@@ -4694,6 +4694,25 @@ mod tests {
         assert_eq!(result.rows_updated, if old_server { 0 } else { 5 });
     }
 
+    #[tokio::test]
+    async fn test_alter_columns_rejects_missing_changes_before_request() {
+        let table = Table::new_with_handler::<String>("my_table", |request| {
+            panic!("Unexpected request: {}", request.url().path())
+        });
+
+        for alterations in [
+            vec![ColumnAlteration::new("id".into())],
+            vec![
+                ColumnAlteration::new("id".into()).rename("new_id".into()),
+                ColumnAlteration::new("id".into()),
+            ],
+        ] {
+            let err = table.alter_columns(&alterations).await.unwrap_err();
+            assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+            assert!(err.to_string().contains("path 'id'"));
+        }
+    }
+
     #[rstest]
     #[case(true)]
     #[case(false)]

rust/lancedb/src/table.rs

@@ -1919,11 +1919,28 @@ impl Table {
         self.inner.function_errors(&request).await
     }
 
-    /// Change a column's name or nullability.
+    /// Change a column's name, data type, or nullability.
+    ///
+    /// Each alteration must specify at least one of `rename`, `data_type`, or
+    /// `nullable`. Returns [`Error::InvalidInput`] before applying any changes
+    /// if an alteration does not specify any of these fields.
     pub async fn alter_columns(
         &self,
         alterations: &[ColumnAlteration],
     ) -> Result<AlterColumnsResult> {
+        for alteration in alterations {
+            if alteration.rename.is_none()
+                && alteration.nullable.is_none()
+                && alteration.data_type.is_none()
+            {
+                return Err(Error::InvalidInput {
+                    message: format!(
+                        "One of rename, nullable or data_type must be specified for path '{}'",
+                        alteration.path
+                    ),
+                });
+            }
+        }
         self.inner.alter_columns(alterations).await
     }
 

rust/lancedb/src/table/schema_evolution.rs

@@ -807,6 +807,29 @@ mod tests {
 
     // Alter Columns Tests
 
+    #[tokio::test]
+    async fn test_alter_columns_rejects_missing_changes() {
+        let conn = connect("memory://").execute().await.unwrap();
+        let batch = record_batch!(("id", Int32, [1, 2, 3])).unwrap();
+        let table = conn.create_table("test", batch).execute().await.unwrap();
+        let initial_version = table.version().await.unwrap();
+        let initial_schema = table.schema().await.unwrap();
+
+        for alterations in [
+            vec![ColumnAlteration::new("id".into())],
+            vec![
+                ColumnAlteration::new("id".into()).rename("new_id".into()),
+                ColumnAlteration::new("id".into()),
+            ],
+        ] {
+            let err = table.alter_columns(&alterations).await.unwrap_err();
+            assert!(matches!(err, Error::InvalidInput { .. }), "got {err:?}");
+            assert!(err.to_string().contains("path 'id'"));
+            assert_eq!(table.version().await.unwrap(), initial_version);
+            assert_eq!(table.schema().await.unwrap(), initial_schema);
+        }
+    }
+
     #[tokio::test]
     async fn test_alter_column_rename() {
         let conn = connect("memory://").execute().await.unwrap();