How to pass Python instance to C++ via Python/C API

Question

I'm extending my library with Python (2.7) by wrapping interfaces with SWIG 2.0, and have a graph object in which I want to create a visitor. In C++, the interface looks like this:

    struct Visitor
    {
        virtual void OnStateBegin() = 0;
        virtual void OnNode(Node* n) = 0;
        virtual void OnStateEnd() = 0;
    };

I would like to define a class in Python that does the equivalent, all defined in python, that will allow for the definition of a visitor:

class GraphVisitor:
    def __init__(self, label):
        self._label = label
        print("__init__(self,{0})".format(self._label))
    def OnStateBegin(self):
        print("OnStateBegin()" + self._label)
    def OnNode(self, i_node):
        print("OnNode()" + self._label)
    def OnStateEnd(self):
        print("OnStateEnd()" + self._label)

And what I'm trying to do is create an instance of a GraphVisitor in python script, and call the methods OnStateBegin(), OnNode(), and OnStateEnd() for a given instance from C++. Here's what I'd like to do in Python:

#model is a SWIG wrapped class
mvis = GraphVisitor("This is a test")
model.Visit("mvis") # I'm not sure how to pass the instance 'mvis' to C++?

And in my C++ wrapped by Swig, I'm not sure how to get at the instance 'mvis'? I can call functions defined in Python no problem, but instances has me stumped!

`mvis` is just an instance of a python class. It has noting to do with your `struct Visitor`. In C/C++ you can only access it as a `PyObject*`. — Thomas, Apr 28 '15 at 20:28
I know that. I was trying to describe that I'm trying to define a visitor in python and thats the c++ counterpart. — Dennis Ward, Apr 29 '15 at 00:27
Have you seen my previous answer: http://stackoverflow.com/questions/9040669/how-can-i-implement-a-c-class-in-python-to-be-called-by-c/9042139#9042139 (you can skip the bits about embedding, but the bit about converting to a C++ interface from a PyObject does exactly what you want) — Flexo, Apr 29 '15 at 16:51

score 1 · Answer 1 · answered May 20 '15 at 11:23

In order to solve this problem, I retrieved the class from the module given it's module name and class name (the code below assumes the module hasn't already been loaded):

void Model::Visit(const char* mod_name, const char* class_name)
{
    PyErr_Clear();
    PyObject* mod_name_obj = PyString_FromString(mod_name);
    PyObject* class_name_obj = PyString_FromString(class_name);

    PyObject* py_module = PyImport_Import(mod_name_obj);
    PyObject* err_1 = PyErr_Occurred();
    if(err_1)
        PyErr_Print();

Once I had the module, I looked up the class from it's dictionary:

    if(py_module)
    {
        PyObject* py_module_dict = PyModule_GetDict(py_module);
        PyObject* py_class = PyDict_GetItem(py_module_dict, class_name_obj);

I simplified my problem a bit by instantiating the python class in C++, then created my visitor, and finally visited it:

        if(py_class && PyClass_Check(py_class) && PyCallable_Check(py_class))
        {
            PyObject* inst = PyInstance_New(py_class, 0, 0);

            if(inst && PyInstance_Check(inst))
            {
                IModel::IVisitorPtr py_visitor = new PyModelVisitor(inst);

                _model->Visit(py_visitor);
            }
        }
    }
}

The visitor had 3 functions OnStateBegin(), OnNode(), and OnStateEnd(). I added to my SWIG python binding generator an option to generate a header file for external access to the SWIG runtime with the -external-runtime option, so I could create a class in C++ (INode* below) and pass it to Python as the argument to the python OnNode() member function as follows (error checking removed for brevity):

VisitorCtrl OnNode(INode* node)
{
    Node* node_impl = new NodeImpl(node);
    PyObject* pynode = SWIG_NewPointerObj(node_impl, SWIG_TypeQuery("Node *"), 0);
    PyObject* result = PyObject_CallMethodObjArgs(_inst, PyString_FromString("OnNode"), pynode, 0);

    long rivis = PyInt_AsLong(result);

    return(static_cast<VisitorCtrl>(rivis));
}

score 0 · Answer 2 · answered Apr 29 '15 at 05:32

I don't know if that's possible with SWIG, but you can do it with SIP.

sip_vector_test.h:

class EXPORT Node {
public:
    explicit Node(int n) : n_(n) {};
    int getN() const { return n_; }
private:
    int n_;
};
struct EXPORT NodeVisitor {
    virtual void OnNode(Node* n) = 0;
};
struct EXPORT Graph {
public:
    void addNode(int num);
    void accept(NodeVisitor *nv);
private:
    std::vector< std::shared_ptr<Node> > nodes_;
};

visitor.sip:

%Module pyvisit

%ModuleHeaderCode
#include "sip_visitor_test.h"
%End

class Node {
public:
    explicit Node(int n);
    int getN() const;
};
struct NodeVisitor {
    virtual void OnNode(Node* n) = 0;
};
struct Graph {
public:
    void addNode(int num);
    void accept(NodeVisitor *nv);
};

Using it from Python:

>>> import pyvisit
>>> g = pyvisit.Graph()
>>> g.addNode(3)
>>> g.addNode(5)
>>> class PyNodeVisitor(pyvisit.NodeVisitor):
>>>     def OnNode(self, node):
>>>         print(node.getN())
>>> pnv = PyNodeVisitor()
>>> g.accept(pnv)
3
5

I've put a zip file containing the source code of this test project on my homepage.

Thanks a lot for the answer! I'm not using SIP, and wanted to see if this could be done without adding a new type. — Dennis Ward, Apr 29 '15 at 12:06

How to pass Python instance to C++ via Python/C API

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