Question

I am able to use SWIG_NewPointerObj to pass a C++ object to Python, but it’s not the PyObject* so I can’t use it properly in Python.

My question is: Is there a way to fetch the PyObject* of a C++ object that was created from Python, in C++?

Answer 1

There isn’t a unique PyObject per C++ object. For example if you wrap the following:

struct foo {};

struct foo *test() {
  static struct foo inst;
  return &inst;
}

Every call to test() inside Python will return a new PyObject – SWIG has no easy way to know they’re identical.

The PyObjects that get returned are thin wrappers, i.e. proxies around the C++ object that have no local state. From the Python perspective these all look and behave like the same object because they delegate everything to the C++ object.

If you want to get hold of a PyObject there are ways you can do this, but in general it’s not a good idea because any changes you make to that object won’t be visible from other PyObjects even if they refer to the same C++ object.