From b2ae763254dfc4237e2b281dab192aad12fd8576 Mon Sep 17 00:00:00 2001
From: Armaan Sandhu <74664101+Ar-maan05@users.noreply.github.com>
Date: Thu, 18 Jun 2026 00:28:48 +0530
Subject: [PATCH] fix(python): raise clear TypeError for bare List/Tuple in
 pydantic schema conversion (#3511)

Closes #3502

## Problem

A bare, unparameterised `typing.List` / `typing.Tuple` field crashes
`to_arrow_schema` with an opaque `AttributeError: __args__`:

```python
from typing import Tuple
from lancedb.pydantic import LanceModel

class Doc(LanceModel):
    items: Tuple

Doc.to_arrow_schema()  # AttributeError: __args__
```

In `_py_type_to_arrow_type`, the branch `elif getattr(py_type,
"__origin__", None) in (list, tuple)` is taken for a bare generic (its
`__origin__` is `list / tuple`), but the next line reads
`py_type.__args__[0]`, and a bare generic has no `__args__`. Other
unsupported types (e.g. `Dict[str, int]`) correctly raise a clear
`TypeError`, so this case is inconsistent.

Fix

Guard the element-type lookup with `getattr(py_type, "__args__", None)`
and raise a clear `TypeError` when it is missing, matching the existing
behavior for other unsupported types. Bare builtin list / tuple are
unaffected (their `__origin__` is `None`, so they already fall through
to the existing `TypeError`).

Testing

- Added `test_bare_generic_raises_type_error` covering both `List` and
`Tuple`.
- ruff format and ruff check clean.
---
 python/python/lancedb/pydantic.py    | 13 ++++++++++++-
 python/python/tests/test_pydantic.py | 12 ++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/python/python/lancedb/pydantic.py b/python/python/lancedb/pydantic.py
index 8b1f991f9..c4dedc0e6 100644
--- a/python/python/lancedb/pydantic.py
+++ b/python/python/lancedb/pydantic.py
@@ -275,7 +275,18 @@ def _py_type_to_arrow_type(py_type: Type[Any], field: FieldInfo) -> pa.DataType:
         tz = get_extras(field, "tz")
         return pa.timestamp("us", tz=tz)
     elif getattr(py_type, "__origin__", None) in (list, tuple):
-        child = py_type.__args__[0]
+        # A bare, unparameterised ``typing.List`` / ``typing.Tuple`` matches this
+        # branch (its ``__origin__`` is ``list`` / ``tuple``) but has no
+        # ``__args__``, so we cannot infer the element type. Raise a clear
+        # ``TypeError`` instead of crashing with an opaque ``AttributeError``.
+        args = getattr(py_type, "__args__", None)
+        if not args:
+            raise TypeError(
+                "Converting Pydantic type to Arrow Type: unsupported type "
+                f"{py_type}. Specify the element type, e.g. List[int] instead "
+                "of a bare List."
+            )
+        child = args[0]
         return _pydantic_list_child_to_arrow(child, field)
     raise TypeError(
         f"Converting Pydantic type to Arrow Type: unsupported type {py_type}."
diff --git a/python/python/tests/test_pydantic.py b/python/python/tests/test_pydantic.py
index 701bbef5a..e1d533784 100644
--- a/python/python/tests/test_pydantic.py
+++ b/python/python/tests/test_pydantic.py
@@ -188,6 +188,18 @@ def test_nested_struct_list():
     assert schema == expect_schema
 
 
+def test_bare_generic_raises_type_error():
+    # A bare, unparameterised List/Tuple has no element type to map to Arrow.
+    # It should raise a clear TypeError, not crash with AttributeError: __args__.
+    for bare in (List, Tuple):
+
+        class TestModel(pydantic.BaseModel):
+            items: bare
+
+        with pytest.raises(TypeError, match="unsupported type"):
+            pydantic_to_schema(TestModel)
+
+
 def test_nested_struct_list_optional():
     class SplitInfo(pydantic.BaseModel):
         start_frame: int