To to build a zero-length byte array (`b""`) with Py_BuildValue (Instead it builds `None`)

KerstinKeller · May 15, 2023, 3:00pm

Our middleware eCAL is written in C++ and it has Python bindings. They use Py_BuildValue to create Python objects, see

github.com

eclipse-ecal/ecal/blob/6fc81d074bc3d5918df8378e56c6bb3134a26e95/lang/python/core/src/ecal_wrap.cxx#L592


      
            if (!g_pygil_init)
            {
              g_pygil_init = 1;
              PyEval_InitThreads();
            }
          #endif
          
          
  PyGILState_STATE state = PyGILState_Ensure();
          
          
  PyObject* topic_name = Py_BuildValue("s",  topic_name_);
            PyObject* content    = Py_BuildValue(python_formatter.c_str(), data_->buf, (Py_ssize_t) data_->size);
            PyObject* time       = Py_BuildValue("L",  data_->time);
          
          
  PyObject* args = PyTuple_New(3);
            PyTuple_SetItem(args, 0, topic_name);
            PyTuple_SetItem(args, 1, content);
            PyTuple_SetItem(args, 2, time);
          
          
  PySubscriberCallbackMapT::const_iterator iter = g_subscriber_pycallback_map.find(handle_);
            if (iter != g_subscriber_pycallback_map.end())
            {

In some cases we would like to create a zero-length byte array, using a pointer, a length, and the y# formatter. In these cases, the pointer which is passed to the Py_BuildValue function, is nullptr.
However, this creates a None value, as specified in the documentation. ( Parsing arguments and building values — Python 3.11.3 documentation )

So the question is: how would a be able to build the equivalent to b"" from C/C++? Passing some random non-null value seems a bit strange.
Thanks for the help.

barry-scott · May 15, 2023, 3:25pm

Seems that the caller must set data_->buf to a valid address and the data->size to 0.
If they do that does it work?