如何用PyObjects声明Boost.Python C++类

Question

我想写一个C++类，用来处理PyObjects，这样我就可以从Python中访问它们，并且可以引用一个已经存在的Python对象实例。简单来说，我想在C++类中存储和管理Python对象实例。

举个例子：

struct Var
{
    PyObject *Test_1;
    PyObject *Test_2;

};

#include <boost/python.hpp>
using namespace boost::python;

BOOST_PYTHON_MODULE(Var)
{
    class_<Var>("Var", init<std::string>())
        .def_readwrite("Test_1", &Var::value)
        .def_readwrite("Test_2", &Var::value)
    ;
}

在Python中，我想这样做，如果使用Boost.Python的话，这样是否可行：

class Test:
    def __init__(self, a = 0, b = 2):
        self.a = 0
        self.b = 0


test = Test(2,2)

import Var
newStore = Var
newStore.Test_1 = test

非常感谢！

问候

Flo

Answer 1

在用C++管理Python对象的时候，如果你使用Boost.Python，建议用boost::python::object类，而不是PyObject。这个object就像Python里的变量一样，可以让你的C++代码看起来更像Python的代码。而且，它的行为也很像智能指针，能够自动管理引用计数和对象的生命周期。而使用PyObject的话，你就得手动管理这些引用计数了。

---

下面是一个完整的例子，基于原始代码，展示了如何使用boost::python::object和PyObject：

#include <boost/python.hpp>

/// @brief Mockup type that can manage two Python objects.
struct var
{
  boost::python::object test_1; // managed
  PyObject* test_2;             // must explicitly manage

  var()
    : test_2(Py_None)
  {
    Py_INCREF(test_2);
  }

  ~var()
  {
    Py_DECREF(test_2);
  }
};

/// @brief Auxiliary function used to return a non-borrowed reference to
//         self.test_2.  This is necessary because Boost.Python assumes
//         that PyObject* passed from C++ to Python are not borrowed.
PyObject* var_test_2_getter(const var& self)
{
  PyObject* object = self.test_2;
  Py_INCREF(object);
  return object;
}

/// @brief Auxiliary function used to manage the reference count of
///        objects assigned to var.test_2.
void var_test_2_setter(var& self, PyObject* object)
{
  Py_DECREF(self.test_2);
  self.test_2 = object;
  Py_INCREF(self.test_2);
}

BOOST_PYTHON_MODULE(example)
{
  namespace python = boost::python;
  python::class_<var>("Var", python::init<>())
    .def_readwrite("Test_1", &var::test_1)
    .add_property("Test_2", &var_test_2_getter, &var_test_2_setter)
    ;
}

交互使用：

>>> class Test:
...     def __init__(self, a=0, b=2):
...         self.a = a
...         self.b = b
... 
>>> test = Test(2, 2)
>>> from sys import getrefcount
>>> count = getrefcount(test)
>>> import example
>>> store = example.Var()
>>> store.Test_1 = test
>>> assert(store.Test_1 is test)
>>> assert(count + 1 == getrefcount(test))
>>> assert(store.Test_1.a == 2)
>>> store.Test_1.a = 42
>>> assert(test.a == 42)
>>> store.Test_2 = test
>>> assert(store.Test_2 is test)
>>> assert(count + 2 == getrefcount(test))
>>> assert(count + 2 == getrefcount(store.Test_2))
>>> store.Test_2 = None
>>> assert(count + 1 == getrefcount(test))
>>> store = None
>>> assert(count == getrefcount(test))