Improve coverage for qdp.py by testing import fallback and stub behavior

testing/qumat/test_qdp_module.py

@@ -0,0 +1,91 @@
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.
+# The ASF licenses this file to You under the Apache License, Version 2.0
+# (the "License"); you may not use this file except in compliance with
+# the License.  You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import sys
+import importlib
+import pytest
+from unittest.mock import patch
+
+
+def _reload_qdp_without_extension():
+    """
+    Safely reload qumat.qdp while simulating missing _qdp extension.
+    Restores sys.modules after execution to avoid test pollution.
+    """
+    original_qdp = sys.modules.get("qumat.qdp")
+    original_ext = sys.modules.get("_qdp")
+
+    try:
+        # Remove cached modules
+        sys.modules.pop("qumat.qdp", None)
+        sys.modules.pop("_qdp", None)
+
+        # Simulate missing compiled extension
+        with patch.dict(sys.modules, {"_qdp": None}):
+            module = importlib.import_module("qumat.qdp")
+            return importlib.reload(module)
+
+    finally:
+        # Restore previous state
+        if original_qdp is not None:
+            sys.modules["qumat.qdp"] = original_qdp
+        else:
+            sys.modules.pop("qumat.qdp", None)
+
+        if original_ext is not None:
+            sys.modules["_qdp"] = original_ext
+        else:
+            sys.modules.pop("_qdp", None)
+
+
+def test_qdp_import_fallback_warning():
+    with pytest.warns(ImportWarning):
+        qdp = _reload_qdp_without_extension()
+    assert qdp is not None
+
+
+def test_qdp_engine_stub_raises_import_error():
+    with pytest.warns(ImportWarning):
+        qdp = _reload_qdp_without_extension()
+
+    with pytest.raises(ImportError, match="install"):
+        qdp.QdpEngine()
+
+
+def test_quantum_tensor_stub_raises_import_error():
+    with pytest.warns(ImportWarning):
+        qdp = _reload_qdp_without_extension()
+
+    with pytest.raises(ImportError, match="install"):
+        qdp.QuantumTensor()
+
+
+def test_qdp_all_exports():
+    with pytest.warns(ImportWarning):
+        qdp = _reload_qdp_without_extension()
+
+    assert "QdpEngine" in qdp.__all__
+    assert "QuantumTensor" in qdp.__all__
+
+
+def test_make_stub_direct_call():
+    with pytest.warns(ImportWarning):
+        qdp = _reload_qdp_without_extension()
+
+    StubClass = qdp._make_stub("TestStub")
+
+    with pytest.raises(ImportError, match="install"):
+        StubClass()