如何通过Python / C API将Python实例传递给C ++ [英] How to pass Python instance to C++ via Python/C API
问题描述
我使用Python(2.7)扩展我的库,通过包装SWIG 2.0的接口,并有一个图形对象,其中我想创建一个访问者。在C ++中,界面如下所示:
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;
};
我想在Python中定义一个类,它在python中定义,允许定义访问者:
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)
我要做的是创建一个python脚本中的GraphVisitor实例,并为C ++中给定的实例调用OnStateBegin(),OnNode()和OnStateEnd()方法。这里是我想在Python做的:
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++?
在我的Swift包装的C ++中,我不知道如何获取实例mvis '?我可以调用在Python中定义的函数没有问题,但是实例让我陷入困境!
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!
推荐答案
为了解决这个问题,模块中的类给出了它的模块名和类名(以下代码假定模块尚未加载):
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();
一旦我有了这个模块,我就从它的字典中查找类:
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);
我通过在C ++中实例化python类来简化我的问题,然后创建我的访问者访问它:
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);
}
}
}
}
visitor有3个函数OnStateBegin(),OnNode()和OnStateEnd()。我添加到我的SWIG python绑定生成器一个选项生成头文件,用于外部访问SWIG运行时与 - 外部运行时选项,所以我可以在C + +(INode *下面)创建一个类,并将其传递给Python作为参数python OnNode函数如下(为简洁而删除错误检查):
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));
}
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