fix: handle empty list with schema in table creation (#2548)

## Summary
Fixes IndexError when creating tables with empty list data and a
provided schema. Previously, `_into_pyarrow_reader()` would attempt to
access `data[0]` on empty lists, causing an IndexError. Now properly
handles empty lists by using the provided schema.

Also adds regression tests for GitHub issues #1968 and #303 to prevent
future regressions with empty table scenarios.

## Changes
- Fix IndexError in `_into_pyarrow_reader()` for empty list + schema
case
- Add Optional[pa.Schema] parameter to handle empty data gracefully  
- Add `test_create_table_empty_list_with_schema` for the IndexError fix
- Add `test_create_empty_then_add_data` for issue #1968
- Add `test_search_empty_table` for issue #303

## Test plan
- [x] All new regression tests pass
- [x] Existing tests continue to pass
- [x] Code formatted with `make format`

python/python/lancedb/table.py

@@ -102,7 +102,9 @@ if TYPE_CHECKING:
     )
 
 
-def _into_pyarrow_reader(data) -> pa.RecordBatchReader:
+def _into_pyarrow_reader(
+    data, schema: Optional[pa.Schema] = None
+) -> pa.RecordBatchReader:
     from lancedb.dependencies import datasets
 
     if _check_for_hugging_face(data):
@@ -123,6 +125,12 @@ def _into_pyarrow_reader(data) -> pa.RecordBatchReader:
         raise ValueError("Cannot add a single dictionary to a table. Use a list.")
 
     if isinstance(data, list):
+        # Handle empty list case
+        if not data:
+            if schema is None:
+                raise ValueError("Cannot create table from empty list without a schema")
+            return pa.Table.from_pylist(data, schema=schema).to_reader()
+
         # convert to list of dict if data is a bunch of LanceModels
         if isinstance(data[0], LanceModel):
             schema = data[0].__class__.to_arrow_schema()
@@ -236,7 +244,7 @@ def _sanitize_data(
     # 1. There might be embedding columns missing that will be added
     #    in the add_embeddings step.
     # 2. If `allow_subschemas` is True, there might be columns missing.
-    reader = _into_pyarrow_reader(data)
+    reader = _into_pyarrow_reader(data, target_schema)
 
     reader = _append_vector_columns(reader, target_schema, metadata=metadata)
 

python/python/tests/test_query.py

@@ -1339,3 +1339,20 @@ async def test_query_timeout_async(tmp_path):
             .nearest_to([0.0, 0.0])
             .to_list(timeout=timedelta(0))
         )
+
+
+def test_search_empty_table(mem_db):
+    """Test searching on empty table should not crash
+
+    Regression test for issue #303:
+    https://github.com/lancedb/lancedb/issues/303
+    Searching on empty table produces scary error message
+    """
+    schema = pa.schema(
+        [pa.field("vector", pa.list_(pa.float32(), 2)), pa.field("id", pa.int64())]
+    )
+    table = mem_db.create_table("test_empty_search", schema=schema)
+
+    # Search on empty table should return empty results, not crash
+    results = table.search([1.0, 2.0]).limit(5).to_list()
+    assert results == []

python/python/tests/test_table.py

@@ -1804,3 +1804,45 @@ def test_stats(mem_db: DBConnection):
             },
         },
     }
+
+
+def test_create_table_empty_list_with_schema(mem_db: DBConnection):
+    """Test creating table with empty list data and schema
+
+    Regression test for IndexError: list index out of range
+    when calling create_table(name, data=[], schema=schema)
+    """
+    schema = pa.schema(
+        [pa.field("vector", pa.list_(pa.float32(), 2)), pa.field("id", pa.int64())]
+    )
+    table = mem_db.create_table("test_empty_list", data=[], schema=schema)
+    assert table.count_rows() == 0
+    assert table.schema == schema
+
+
+def test_create_table_empty_list_no_schema_error(mem_db: DBConnection):
+    """Test that creating table with empty list and no schema raises error"""
+    with pytest.raises(
+        ValueError, match="Cannot create table from empty list without a schema"
+    ):
+        mem_db.create_table("test_empty_no_schema", data=[])
+
+
+def test_add_table_with_empty_embeddings(tmp_path):
+    """Test exact scenario from issue #1968
+
+    Regression test for issue #1968:
+    https://github.com/lancedb/lancedb/issues/1968
+    """
+    db = lancedb.connect(tmp_path)
+
+    class MySchema(LanceModel):
+        text: str
+        embedding: Vector(16)
+
+    table = db.create_table("test", schema=MySchema)
+    table.add(
+        [{"text": "bar", "embedding": [0.1] * 16}],
+        on_bad_vectors="drop",
+    )
+    assert table.count_rows() == 1