How can I use PyObject_IsInstance with a non-builtin class as second argument?

Question

In C/C++, I want to see if a PyObject is an instance. Unfortunately, the PyInstance_Check macro doesn't work with new-style classes.

So, according to forums posts that I read, PyObject_IsInstance could solve the problem. However, all the examples that I have found are demonstrating comparisons with built-in types, like ints and strings.

I want to know how I can construct a PyObject that represents a class of a type, so I can pass it to the second argument of PyObject_IsInstance . Can you help me?

Answer 1

I have finally found the answer myself, based on the contents of that page: Create instance of a python class , declared in python, with C API

// PyInstance_Check works only for old-style classes.
qBool isInstance = PyInstance_Check(pyInstance);
if (!isInstance)
{
    // It might be an instance of a new-style class.
    //
    PyRef module = PyImport_ImportModule("module.where.myclass.is");
    PyObject* moduleDict = PyModule_GetDict(module.Get());
    PyObject* protocolClass = PyDict_GetItemString(moduleDict, "MyClass");

    int returnValue = PyObject_IsInstance(pyObject, protocolClass);
}

Answer 2

You could use the corresponding PyTypeObject if it is defined in the same module. If the type object (let's say it's called ProtocolType ) is not visible from where you want to use PyObject_IsInstance , first declare a prototype with:

extern PyTypeObject ProtocolType;

And then use it like this:

PyObject_IsInstance(object, reinterpret_cast<PyObject*>(&ProtocolType))