Mailman 3 October 2014 - Cplusplus-sig

Boost.Python and thread safety
by Davidson, Josh Oct. 29, 2014

Oct. 29, 2014

Hi all, I'm trying to determine if the following constitutes a thread safety violation with Python. To simplify the discussion, let's say have the following fictional classes: * Teacher - Fully implemented in Python * Student - Implemented in C++ with Boost.Python bindings * Quiz - Implemented in C++ with Boost.Python bindings In a Python interpreter, a single instance of a teacher is instantiated and multiple instances of students are instantiated (via the bindings). The teacher then instantiates a number of quiz objects (via the bindings) and invokes a method on the student class that takes a reference to a quiz object. The teacher objects kicks off a Python thread that blocks until all students are completed with the quiz. The student objects set attributes on their assigned quiz objects until they are done. However, if during the course of taking the test, a student kicks of a C++ (Boost) thread, say to scan his classmates' papers, and in that non-Python thread without the GIL, invokes getters/setters on the quiz object, is that a violation of thread safety? The quiz object is protected from concurrent access. If the quiz object was instantiated from a C++ application and passed in there would definitely be no problem. If the quiz object is instantiated via the bindings and passed into the C++ student, I'm not so sure since there would be a thread accessing a (wrapped) Python object from threads that don't hold the GIL. Thanks, Josh

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sharing code between different python extensions
by Wintersberger, Eugen Oct. 28, 2014

Oct. 28, 2014

Hi there I have a little problem with two Python extensions (C++ with boost::python) where share code. An example of what I am trying to do is attached to this mail so I will refer to the code in the tarball to explain the problem. I have to Python extensions ('a' and 'b') each of them exporting two functions. What I want is to use a function exported by 'a' in the code of 'b'. Though the package compiles I get an unresolved symbol error when trying to load 'b'. The reason is quite clear. 'b' cannot resolve the symbol as it is provided by 'a' to which it is not linked. Is it somehow possible with distutils to link the two extensions to that 'b' can see the symbols provided by 'a'? best regards Eugen Wintersberger

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Exposing simple polymorphism with boost python
by Erik Türke Oct. 24, 2014

Oct. 24, 2014

Hi Python-Experts :-) i am starting to get really frustrated trying to expose a simple C++ polymorphism to python with boost::python. I do have the following structure in C++: struct Base { int typeID; }; struct Derived : public Base { int derivedProperty; } //and some more from base derived types.... Base *returnSomethingDerivedFromBase(...) { Derived *ret = new Derived; ret->derivedProperty = 1234; return ret; } BOOST_PYTHON_MODULE(foo) { class_<Base>("Base") .add_property("baseProperty", &Base::baseProperty); class_<Derived, bases<Base> >("Derived") .add_property("derivedProperty", &Derived::derivedProperty); def("returnSomethingDerivedFromBase", returnSomethingDerivedFromBase); } And in Python i just want to have the following: object = returnSomethingFromDerived() #object is of type Base if object.typeID = 1: #here i want to cast to Derived and access "derivedProperty" #but this is not working :-( : object.__class__ = Derived Is there a way to accomplish this at all? Or isn´t this possible as it would be in C++ ? Thanks a lot for your help!!

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boost python class member getter/setter same name different only by constness
by MM Oct. 3, 2014

Oct. 3, 2014

Hi class C { public: const std::string& name() const; std::string& name(); private: std::string name_; }; given this class C, how would I expose it to python with the class property name? class_<C>("C"). .add_property("name", &C::name, &C::name); or do I use 2 mem function pointers to distinguish the 2 names, and pass those 2 pointers? MM

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boost::python embedding error - runtime - Mac OS X - 1.38
by Igor Karpov Oct. 1, 2014

Oct. 1, 2014

Hi, I am trying to run the following program: #include <iostream> #include <Python.h> #include <boost/python.hpp> using namespace boost::python; using namespace std; int main(int argc, char** argv) { { // Using Python/C Py_Initialize(); PyObject* main_module = PyImport_ImportModule("__main__"); PyObject* globals = PyEval_GetGlobals(); PyObject* locals = PyEval_GetLocals(); PyRun_SimpleString("print 'Hello World, from Python/C!'\n"); Py_Finalize(); } { // Using boost::python Py_Initialize(); object main_module = import("__main__"); object main_namespace = main_module.attr("__dict__"); try { object ignored = exec("print 'Hello World, from boost::python!'\n", main_namespace); Py_Finalize(); } catch (error_already_set const& e) { PyErr_Print(); return 1; } } return 0; } on Linux (both x86 and x86_64), with Ubuntu's default version of Boost (1.34.1) installed, I am able to do so as expected: $ ./embed Hello World, from Python/C! Hello World, from boost::python! on Mac OS X, I get a runtime error: $ ./embed Hello World, from Python/C! Bus Error Running with gdb, this is the stack trace: Hello World, from Python/C! Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: KERN_PROTECTION_FAILURE at address: 0x00000000 0x00000000 in ?? () (gdb) where #0 0x00000000 in ?? () #1 0x00bb618f in PyEval_GetGlobals () #2 0x00bce0dd in PyImport_Import () #3 0x00bce2b0 in PyImport_ImportModule () #4 0x008127d4 in boost::python::import (name=@0xbffff4a8) at libs/python/src/import.cpp:20 #5 0x00004446 in main (argc=1, argv=0xbffff510) at embed.cc:22 For this Mac, boost version is latest (1.38) and python version is: Python 2.5.4 (r254:67917, Dec 23 2008, 14:57:27) [GCC 4.0.1 (Apple Computer, Inc. build 5363)] on darwin Is this a bug? I attached the source and the cmake file I am using to build, for convenience. Thanks, --Igor.

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