Fix SequenceEmpty dtype parameter ignored in eager mode

onnxscript/_internal/evaluator.py

@@ -251,13 +251,25 @@ def eval_op(
         args: Sequence[ExtendedModeValue],
         kwargs: Mapping[str, ExtendedModeValue],
     ):
+        from onnxscript._internal.typed_sequence import (
+            _TypedSequence,  # pylint: disable=import-outside-toplevel
+        )
+
         op_signature = op.op_signature
         assert op_signature is not None, f"Op {op.name} has no signature."
         attributes = _unwrap_tensors_in_kwargs(kwargs)
         attributes, closure = self._adapt_attributes(op_signature, attributes)
         inputs = self._adapt_inputs(op_signature, args)
         outputs = self._eval(op.op_schema, inputs, attributes, closure)
-        return self._adapt_outputs(outputs)
+        result = self._adapt_outputs(outputs)
+
+        # Handle SequenceEmpty: wrap result with _TypedSequence to preserve dtype
+        # for empty sequences where type cannot be inferred from elements.
+        if op.name == "SequenceEmpty":
+            dtype = attributes.get("dtype") or onnx.TensorProto.FLOAT
+            return _TypedSequence(dtype, result)
+
+        return result
 
     def eval_function(
         self,
@@ -364,8 +376,15 @@ def compute_num_outputs(
 
 def _onnxscript_to_numpy_value(v):
     """Converts an onnxscript encoding of an ONNX value into the numpy encoding used by runtimes."""
+    from onnxscript._internal.typed_sequence import (
+        _TypedSequence,  # pylint: disable=import-outside-toplevel
+    )
+
     if isinstance(v, tensor.Tensor):
         return v.value
+    # Handle _TypedSequence BEFORE generic list check to preserve dtype info
+    if isinstance(v, _TypedSequence):
+        return _TypedSequence(v.onnx_dtype, [_onnxscript_to_numpy_value(x) for x in v])
     if isinstance(v, list):
         return [_onnxscript_to_numpy_value(x) for x in v]
     if isinstance(v, tuple):

onnxscript/_internal/typed_sequence.py

@@ -0,0 +1,33 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+from __future__ import annotations
+
+from typing import Any, Iterable
+
+
+class _TypedSequence(list):
+    """A list subclass that preserves ONNX dtype information.
+
+    ONNX sequences need to carry element type information, but Python lists
+    don't have this natively. This class wraps a list with dtype info, which is
+    especially important for empty sequences where type cannot be inferred.
+
+    Example::
+        seq = _TypedSequence(onnx.TensorProto.FLOAT16)
+        # seq is now an empty list that knows it should contain FLOAT16 tensors
+    """
+
+    __slots__ = ("_onnx_dtype",)
+
+    def __init__(self, dtype: int, iterable: Iterable[Any] | None = None) -> None:
+        super().__init__(iterable if iterable is not None else ())
+        self._onnx_dtype = dtype
+
+    @property
+    def onnx_dtype(self) -> int:
+        """Return the ONNX TensorProto data type for sequence elements."""
+        return self._onnx_dtype
+
+    def __repr__(self) -> str:
+        return f"_TypedSequence(dtype={self._onnx_dtype}, {list(self)})"

onnxscript/_internal/utils.py

@@ -10,6 +10,7 @@
 import onnx_ir as ir
 
 from onnxscript import tensor
+from onnxscript._internal.typed_sequence import _TypedSequence
 
 
 def external_tensor(
@@ -72,6 +73,13 @@ def value_to_type_proto(val):
         return onnx.helper.make_tensor_type_proto(onnx.TensorProto.INT32, [])  # noqa: TID251
     if isinstance(val, (float, np.float32)):
         return onnx.helper.make_tensor_type_proto(onnx.TensorProto.FLOAT, [])  # noqa: TID251
+
+    # Handle _TypedSequence BEFORE generic list check
+    if isinstance(val, _TypedSequence):
+        return onnx.helper.make_sequence_type_proto(  # noqa: TID251
+            onnx.helper.make_tensor_type_proto(val.onnx_dtype, None)  # noqa: TID251
+        )
+
     if isinstance(val, list):
         if len(val) > 0:
             return onnx.helper.make_sequence_type_proto(value_to_type_proto(val[0]))  # noqa: TID251

tests/eager_mode_test.py

@@ -4,6 +4,7 @@
 import unittest
 
 import numpy as np
+import onnx
 import parameterized
 
 import onnxscript.evaluator
@@ -51,6 +52,49 @@ def Concat(seq):
         output2 = Concat([os_tensor, os_tensor])
         self.assertIsInstance(output2, onnxscript.tensor.Tensor)
 
+    def test_sequence_empty_preserves_dtype(self):
+        """Regression test for SequenceEmpty dtype parameter.
+
+        Verify that SequenceEmpty correctly preserves the dtype attribute
+        so that SequenceInsert doesn't fail with type mismatch errors.
+        """
+
+        @script()
+        def test_float16(img_in):
+            seq = op.SequenceEmpty(dtype=onnx.TensorProto.FLOAT16)
+            seq = op.SequenceInsert(seq, img_in)
+            return seq
+
+        @script()
+        def test_double(img_in):
+            seq = op.SequenceEmpty(dtype=onnx.TensorProto.DOUBLE)
+            seq = op.SequenceInsert(seq, img_in)
+            return seq
+
+        @script()
+        def test_int64(img_in):
+            seq = op.SequenceEmpty(dtype=onnx.TensorProto.INT64)
+            seq = op.SequenceInsert(seq, img_in)
+            return seq
+
+        # Test FLOAT16
+        img_float16 = np.random.randn(2, 3).astype(np.float16)
+        res = test_float16(img_float16)
+        self.assertEqual(len(res), 1)
+        np.testing.assert_array_equal(res[0], img_float16)
+
+        # Test DOUBLE
+        img_double = np.random.randn(2, 3).astype(np.float64)
+        res = test_double(img_double)
+        self.assertEqual(len(res), 1)
+        np.testing.assert_array_equal(res[0], img_double)
+
+        # Test INT64
+        img_int64 = np.array([[1, 2], [3, 4]], dtype=np.int64)
+        res = test_int64(img_int64)
+        self.assertEqual(len(res), 1)
+        np.testing.assert_array_equal(res[0], img_int64)
+
 
 @script()
 def add_with_alpha(this, other, alpha: float = 1.0):