Remove exception handling of PyObject* return values in CLIF callbacks

If an exception has occurred in the py-callback (-> `PyObject* = nullptr` is returned) the caller must handle the exception. This change removes the exception handling from `ReturnValue<PyObject*>` and offloads this responsibility on the caller. Previously, the exception signal was cleared in the call to `HandlePyExc()` which prohibits the user from handling the exception, while still returning `nullptr` and implicitly signalling that there was an error.

Tested: `blaze test third_party/clif/... devtools/clif/...` and
`tap_presubmit -p all --train -c 493233664`: https://test.corp.google.com/OCL:493233664:BASE:493182079:1670319490310:bf766ad9 + rerun https://test.corp.google.com/OCL:493233664:BASE:493182079:1670337211526:88c1b8f1
PiperOrigin-RevId: 493571121

Author: CLIF Team

clif/python/stltypes.h

@@ -379,9 +379,7 @@ template <>
 class ReturnValue<PyObject*> {
  public:
   PyObject* FromPyValue(PyObject* result) {
-    if (PyErr_Occurred()) {
-      HandlePyExc();
-    }
+    // Return as-is and offload exception handling to the caller.
     return result;
   }
 };

clif/testing/callback.h

@@ -16,6 +16,8 @@
 #ifndef CLIF_TESTING_CALLBACK_H_
 #define CLIF_TESTING_CALLBACK_H_
 
+#include <Python.h>
+
 #include <functional>
 #include <string>
 #include <vector>
@@ -82,4 +84,6 @@ inline std::string LambdaCallback(std::function<std::string()> f) {
   return f();
 }
 
+inline PyObject* PyObjectCallback(std::function<PyObject*()> f) { return f(); }
+
 #endif  // CLIF_TESTING_CALLBACK_H_

clif/testing/python/callback.clif

@@ -31,3 +31,5 @@ from "clif/testing/callback.h":
 
   # Testing pybind11 code generator generating lambda expressions with callbacks
   def LambdaCallback(cb: () -> str) -> bytes
+
+  def PyObjectCallback(cb: () -> object) -> object

clif/testing/python/callback_test.py

@@ -39,6 +39,10 @@ def StringCallback():
   return 'abcd'
 
 
+def RaisingCallback():
+  raise ValueError('raised a value error')
+
+
 class CallbackTest(absltest.TestCase):
 
   def CallbackMethod(self, data):
@@ -90,6 +94,15 @@ def testBytesCallback(self):
     self.assertIsInstance(res, bytes)
     self.assertEqual(res, b'abcd')
 
+  def testPyObjectCallback(self):
+    res = callback.PyObjectCallback(StringCallback)
+    self.assertIsInstance(res, str)
+    self.assertEqual(res, 'abcd')
+
+  def testPyObjectCallbackRaises(self):
+    with self.assertRaisesWithLiteralMatch(ValueError, 'raised a value error'):
+      callback.PyObjectCallback(RaisingCallback)
+
 
 if __name__ == '__main__':
   absltest.main()