take this code
#include<boost/python>
namespace bp = boost::python;
bp::list py_points; //initial list
other_class* C; // this class have a bp::list attribute called py_list
// ... some code ....
// in this part C.py_list.ptr() is 0x0
other_class->py_list = py_list; // problem here!!
the problem is with the operator "="
in the debugger in the object_core.hpp file, this is a boost python core file
inline api::object_base& api::object_base::operator=(api::object_base const& rhs)
{
Py_INCREF(rhs.m_ptr);
Py_DECREF(this->m_ptr); // in this line the program fail
this->m_ptr = rhs.m_ptr;
return *this;
}
what is the proper way to use the operator "="
edited
the problem is the stack, if the pointer other_class->py_list is null (or None because the class constructor is not called) the program can´t call the función Py_DECREF (Don't exist references before of the NULL pointer)
the problem is fix calling the constructor
other_class* C = new othe_class(); // fixed!!
The problem is not the assignment operator, the problem is that py_list's internal PyObject pointer is a nullptr. In the majority of cases, the pointer should not be null. From a Python point of view, it should at least manage a reference to the Python None object, as it done by a default constructed boost::python::object. The default constructor for boost::python::list creates a new empty list. Hence, the source of the problem likely resides within either other_class's constructor or the "some code" block.
To elaborate on the question posed within the title, creating a reference or copying list in Boost.Python is the same as in Python:
The assignment operator will create a reference to a list.
>>> a = ['spam']
>>> b = a
>>> b
['spam']
>>> a is b
True
One can slice a list to create a shallow copy.
>>> a = ['spam']
>>> b = a[:]
>>> b
['spam']
>>> a is b
False
Here is a complete example with the Python equivalent code annotated in the comments.
#include <iostream>
#include <boost/python.hpp>
/// @brief Mockup class.
struct other_class
{
boost::python::list py_list;
};
/// @brief Helper function to print object id and its string representation.
std::string to_string(boost::python::object& o)
{
std::stringstream stream;
stream << o.ptr() << " = "
<< boost::python::extract<std::string>(o.attr("__str__")())();
return stream.str();
}
int main()
{
using std::cout;
using std::endl;
namespace python = boost::python;
Py_Initialize();
try
{
python::object object; // object = None
cout << to_string(object) << "\n" // print object
<< " is none check: " << object.is_none() // print object is None
<< endl;
// Create other_class and populate its list.
other_class* c = new other_class(); // py_list = []
cout << "c->py_list: " << to_string(c->py_list) // print py_list
<< endl;
c->py_list.append("spam"); // py_list.append("spam")
cout << "c->py_list: " << to_string(c->py_list) // print py_list
<< endl;
// Have list1 reference c->py_list.
python::list list1; // list1 = []
cout << "list1: " << to_string(list1) << "\n" // print list1
<< "assign py_list to list1" << endl;
list1 = c->py_list; // list1 = py_list
cout << "list1: " << to_string(list1) << endl; // print list1
// Modify list1 and observe effects on pylist.
cout << "modify list1" << endl;
list1.append(42); // list1.append(42)
cout << "c->py_list: " << to_string(c->py_list) // print py_list
<< endl;
// Shallow-copy list1.
cout << "copying list1 into list2" << endl;
python::list list2(
list1.slice(python::_, python::_)); // list2 = list1[:]
list2.append("eggs"); // list2.append("eggs")
cout << "list2: " << to_string(list2) << "\n" // print list2
<< "list1: " << to_string(list1) << endl; // print list1
delete c;
}
catch (python::error_already_set&)
{
PyErr_Print();
}
}
Output:
0x804e1ac = None
is none check: 1
c->py_list: 0xb707024c = []
c->py_list: 0xb707024c = ['spam']
list1: 0xb70da98c = []
assign py_list to list1
list1: 0xb707024c = ['spam']
modify list1
c->py_list: 0xb707024c = ['spam', 42]
copying list1 into list2
list2: 0xb707cb0c = ['spam', 42, 'eggs']
list1: 0xb707024c = ['spam', 42]
A few points to note in the output:
boost::python::list objects manage a reference to an empty list. 0x804e1ac is None, and none of the list object's internal PyObject pointer manage a reference to it.list1 = py_list assignment causes list1 to manage a reference to the same list managed by py_list. This is exhibited in the output by list1 initially managing a reference to 0xb70da98c, but post-assignment, it manages a reference to 0xb707024c. With list1 and py_list managing the same list, a change to the list through one handle can be observed in the other handle.PyObject internal point for list2 manages a different reference (0xb707cb0c) than list1's pointer (0xb707024c).