From 0b20965d0994a043b7ed0ca84a65822e9e5388d7 Mon Sep 17 00:00:00 2001
From: Minh Vu <vuhoangminh97@gmail.com>
Date: Mon, 2 Nov 2026 13:04:45 +0100
Subject: [PATCH] fix(python)!: reject mixed-value enums in schemas

A mixed-value Pydantic Enum previously produced a UTF-8 Arrow schema even when some member values were non-string, causing writes to fail after schema construction. `pydantic_to_schema` now rejects actual member values with more than one type using `TypeError`, including direct, optional, list, and nested fields. Homogeneous string and integer enums and empty enums retain their mappings, and existing plain-Enum writes remain supported. This intentionally changes mixed-value enums from failing at write time to failing at schema construction.

Validation on the exact head: the Python suite passed with 3034 passed, 126 skipped, and 70 deselected, including 12 mixed-value container cases, the empty-enum case, homogeneous schema coverage, and real plain-Enum writes. All applicable exact-head checks and workflows completed successfully.
---
 python/python/lancedb/pydantic.py    |  9 +++++++--
 python/python/tests/test_pydantic.py | 30 ++++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/python/python/lancedb/pydantic.py b/python/python/lancedb/pydantic.py
index d2b5cb22d..cc3070b72 100644
--- a/python/python/lancedb/pydantic.py
+++ b/python/python/lancedb/pydantic.py
@@ -304,8 +304,12 @@ def _pydantic_type_to_arrow_type(tp: Any, field: FieldInfo) -> pa.DataType:
             # Map Enum to the Arrow type of its value.
             # For string-valued enums, use dictionary encoding for efficiency.
             # For integer enums, use the native type.
-            # Fall back to utf8 for mixed-type or empty enums.
             value_types = {type(m.value) for m in tp}
+            if len(value_types) > 1:
+                raise TypeError(
+                    f"Converting Pydantic Enum {tp.__name__} to Arrow Type: "
+                    "all enum values must have the same type."
+                )
             if len(value_types) == 1:
                 value_type = value_types.pop()
                 if value_type is str:
@@ -435,7 +439,8 @@ def pydantic_to_schema(model: Type[pydantic.BaseModel]) -> pa.Schema:
         an Arrow list, so all its elements must have the same type, as in
         ``Tuple[int, int]`` or ``Tuple[float, ...]``; for a fixed set of fields
         with different types, such as ``Tuple[int, str]``, use a nested
-        ``LanceModel`` (an Arrow struct) instead.
+        ``LanceModel`` (an Arrow struct) instead. Enum values must also all
+        have the same type.
 
     Examples
     --------
diff --git a/python/python/tests/test_pydantic.py b/python/python/tests/test_pydantic.py
index 2a874b68e..eb6962131 100644
--- a/python/python/tests/test_pydantic.py
+++ b/python/python/tests/test_pydantic.py
@@ -1117,6 +1117,36 @@ def test_enum_types():
     assert schema.field("opt_status").nullable
 
 
+@pytest.mark.parametrize("container", ["field", "optional", "list", "nested"])
+@pytest.mark.parametrize("values", [("text", 7), (1, 2.5), (False, "no")])
+def test_mixed_enum_values_are_rejected(container, values):
+    Mixed = Enum("Mixed", {"FIRST": values[0], "SECOND": values[1]})
+
+    class Nested(pydantic.BaseModel):
+        value: Mixed
+
+    annotation = {
+        "field": Mixed,
+        "optional": Optional[Mixed],
+        "list": List[Mixed],
+        "nested": Nested,
+    }[container]
+    model = pydantic.create_model("Model", value=(annotation, ...))
+
+    with pytest.raises(TypeError, match="Enum.*Mixed.*same type"):
+        pydantic_to_schema(model)
+
+
+def test_empty_enum_schema_is_unchanged():
+    class Empty(Enum):
+        pass
+
+    class Model(pydantic.BaseModel):
+        value: Empty
+
+    assert pydantic_to_schema(Model).field("value").type == pa.utf8()
+
+
 def test_plain_enum_values_are_written(mem_db):
     """Members of an Enum without a str/int mixin are written as their values."""