From 528f02ea5eb1035098d8349ff3a2452e03ca1a6a Mon Sep 17 00:00:00 2001
From: uditDewan <udit.dewan21@gmail.com>
Date: Sun, 5 Jul 2026 00:17:56 -0400
Subject: [PATCH] fix(ops): make PairRolling input assert actually check its
 inputs

The assert in PairRolling._load_internal was vacuous: it evaluated
any([isinstance(feature_left, Expression), feature_right, Expression]),
where the bare Expression class object is always truthy, so the check
could never fail. Two non-Expression inputs slipped through and later
died with AttributeError ("'float' object has no attribute 'rolling'").

Check isinstance on both sides so invalid inputs fail fast with the
intended message.
---
 qlib/data/ops.py                      |  4 +--
 tests/ops/test_pair_rolling_inputs.py | 50 +++++++++++++++++++++++++++
 2 files changed, 52 insertions(+), 2 deletions(-)
 create mode 100644 tests/ops/test_pair_rolling_inputs.py

diff --git a/qlib/data/ops.py b/qlib/data/ops.py
index d9a2ffbb3e3..920df45448f 100644
--- a/qlib/data/ops.py
+++ b/qlib/data/ops.py
@@ -1413,8 +1413,8 @@ def __str__(self):
         return "{}({},{},{})".format(type(self).__name__, self.feature_left, self.feature_right, self.N)
 
     def _load_internal(self, instrument, start_index, end_index, *args):
-        assert any(
-            [isinstance(self.feature_left, Expression), self.feature_right, Expression]
+        assert isinstance(self.feature_left, Expression) or isinstance(
+            self.feature_right, Expression
         ), "at least one of two inputs is Expression instance"
 
         if isinstance(self.feature_left, Expression):
diff --git a/tests/ops/test_pair_rolling_inputs.py b/tests/ops/test_pair_rolling_inputs.py
new file mode 100644
index 00000000000..605fc85bea6
--- /dev/null
+++ b/tests/ops/test_pair_rolling_inputs.py
@@ -0,0 +1,50 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import unittest
+
+import numpy as np
+import pandas as pd
+
+from qlib.data.base import Expression
+from qlib.data.ops import Cov
+
+
+class StubFeature(Expression):
+    """Minimal expression returning a fixed series, so operators can be tested without a data provider."""
+
+    def __init__(self, values, name):
+        self._values = list(values)
+        self._name = name
+
+    def __str__(self):
+        # unique per stub: Expression.load caches series keyed on str(self)
+        return self._name
+
+    def _load_internal(self, instrument, start_index, end_index, *args):
+        return pd.Series(self._values, dtype=float)
+
+    def get_longest_back_rolling(self):
+        return 0
+
+    def get_extended_window_size(self):
+        return 0, 0
+
+
+class TestPairRollingInputCheck(unittest.TestCase):
+    """PairRolling must reject two non-Expression inputs instead of failing later with AttributeError."""
+
+    def test_two_numeric_inputs_rejected(self):
+        with self.assertRaises(AssertionError):
+            Cov(1.0, 2.0, 3).load("inst", 0, 4, "day")
+
+    def test_expression_inputs_accepted(self):
+        left = [1.0, 2.0, 3.0, 4.0, 5.0]
+        right = [2.0, 1.0, 4.0, 3.0, 6.0]
+        res = Cov(StubFeature(left, "cov_l"), StubFeature(right, "cov_r"), 3).load("inst", 0, 4, "day")
+        expected = pd.Series(left).rolling(3, min_periods=1).cov(pd.Series(right))
+        np.testing.assert_allclose(res.values, expected.values)
+
+
+if __name__ == "__main__":
+    unittest.main()