cleanup

cpp/src/arrow/python/common.h

@@ -91,7 +91,9 @@ class ARROW_EXPORT OwnedRef {
   PyObject* obj_;
 };
 
-// This assumes that the GIL is held by the caller
+// This is different from OwnedRef in that it assumes that
+// the GIL is held by the caller and doesn't decrement the
+// reference count when release is called.
 class ARROW_EXPORT ScopedRef {
  public:
   ScopedRef() : obj_(nullptr) {}
@@ -102,6 +104,11 @@ class ARROW_EXPORT ScopedRef {
     Py_XDECREF(obj_);
   }
 
+  void reset(PyObject* obj) {
+    Py_XDECREF(obj_);
+    obj_ = obj;
+  }
+
   PyObject* release() {
     PyObject* result = obj_;
     obj_ = nullptr;

cpp/src/arrow/python/python_to_arrow.cc

@@ -283,19 +283,12 @@ Status SerializeSequences(std::vector<PyObject*> sequences, int32_t recursion_de
   SequenceBuilder builder(nullptr);
   std::vector<PyObject *> sublists, subtuples, subdicts;
   for (const auto& sequence : sequences) {
-    PyObject* item;
-    PyObject* iterator = PyObject_GetIter(sequence);
+    ScopedRef iterator(PyObject_GetIter(sequence));
     RETURN_IF_PYERROR();
-    while ((item = PyIter_Next(iterator))) {
-      Status s = Append(item, &builder, &sublists, &subtuples, &subdicts, tensors_out);
-      Py_DECREF(item);
-      // if an error occurs, we need to decrement the reference counts before returning
-      if (!s.ok()) {
-        Py_DECREF(iterator);
-        return s;
-      }
+    ScopedRef item;
+    while (item.reset(PyIter_Next(iterator.get())), item.get()) {
+      RETURN_NOT_OK(Append(item.get(), &builder, &sublists, &subtuples, &subdicts, tensors_out));
     }
-    Py_DECREF(iterator);
   }
   std::shared_ptr<Array> list;
   if (sublists.size() > 0) {