Mailman 3 October 2007 - Cplusplus-sig

Inheritance and __init__
by Bue Vedel-Larsen Oct. 31, 2007

Oct. 31, 2007

I'm having a strange problem with Boost::Python. Consider the following Python code: class Base: def foo(self, str): print str class Derived(Base): def __init__(self): self.foo('__init__') d = Derived() This works as expected: When run, "__init__" is printed. My problem is that if Base is in a C++ module the same code fails with an Boost.Python.ArgumentError. See end of post for example code that exhibit the error. Can anyone point out if I'm doing something wrong? I'm not using any virtual methods, or any kind of advanced inheritance magic. Is this a bug in Boost::Python? Thanks in advance, Bue The exact error is: Boost.Python.ArgumentError: Python argument types in Base.foo(DerivedB, str) did not match C++ signature: foo(class Base {lvalue}, class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >) Minimum example code: #include <boost/python.hpp> #include <string> #include <iostream> class Base { public: Base() {} ~Base() {} void foo( const std::string& str ) { std::cout << "In C++: " << str << std::endl; } }; BOOST_PYTHON_MODULE(test_module) { using namespace boost::python; (new class_< Base >( "Base", "Base class", init<>() )) ->def( "foo", &Base::foo, "method foo" ) ; } void run_script( const std::string& script ) { if( PyRun_SimpleString( script.c_str() ) ) { PyErr_Print(); PyErr_Clear(); } } int main() { PyImport_AppendInittab( "test_module", &inittest_module ); Py_Initialize(); run_script( "from test_module import *\n" ); // This works run_script( "class DerivedA(Base):\n" "\tdef bar(self):\n" "\t\tself.foo('In Python: DerivedA.bar()')\n" "dA = DerivedA()\n" "dA.bar()\n" ); // This doesn't work run_script( "class DerivedB(Base):\n" "\tdef __init__(self):\n" "\t\tself.foo('In Python: DerivedB.__init__()')\n" "dB = DerivedB()\n" ); #ifdef WIN32 system( "PAUSE" ); #endif return 0; }

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C++ singleton
by Emmanuel Taurel Oct. 31, 2007

Oct. 31, 2007

Hello everybody, I am actually writing an interface to python for an existing C++ library using Boost. This library has a C++ singleton class. This class has only one constructor (with arguments) which is "protected" because it's a singleton. I am not able to build a Boost interface to this class because the compiler complains that the constructor I have given to Boost in the class_xxx constructor argument is not public (which is true) ! Therefore, my question is simple : How is it possible to interface such a class with only one constructor which is protected and a static method to create/access the object ? Thank's very much for your answer. Sincerely Emmanuel Taurel (etaurel(a)cells.es) PS : By the way, is there a searchable archive of this mailing list ?

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properties in python class that extend c++ base
by Neal Becker Oct. 30, 2007

Oct. 30, 2007

I have a c++ class with a property: class_<mod_t> ("mod"... .add_property ("SPS", &mod_t::SPS) If I subclass this class in python: class X (mod): Then when python code attempts to access the property in the derived class, it fails: Boost.Python.ArgumentError: Python argument types in None.None(pi4_modulator) did not match C++ signature: None(modulator<non_offset_modbase, double> {lvalue}) Is there a solution to this?

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can't get Boost.Python tutorial's "Hello World" example to work (on linux)
by ted sandler Oct. 30, 2007

Oct. 30, 2007

Hi all, I am running linux, and although the Boost.Python "Quickstart" example works fine, I am not able to get the Boost.Python "Hello World" example working. The quickstart example is located here http://www.boost.org/libs/python/doc/building.html. The hello-world example is located here: http://www.boost.org/libs/python/doc/tutorial/doc/html/python/hello.html. There are problems with the documentation on the hello-world web page (I guess it's out of date). The example files in the Boost-distribution are different from those described in the "hello-world" tutorial. Specifically, the Boost distribution contains a "Jamroot" file while the web page discusses both a "Jamfile" and a "Jamrules" file (whose hyperlink is broken). Anyway, when I cd to the "hello-world" example directory and type bjam at the prompt, bjam fails with the error message I've listed below. Any help getting me up and running here would be great! Thanks a bunch, -Ted Jamroot:17: in modules.load rule python-extension unknown in module Jamfile</home/ted/boost_1_34_1/libs/python/example/tutorial>. /home/ted/boost_1_34_1/tools/build/v2/build/project.jam:312: in load-jamfile /home/ted/boost_1_34_1/tools/build/v2/build/project.jam:68: in load /home/ted/boost_1_34_1/tools/build/v2/build/project.jam:170: in project.find /home/ted/boost_1_34_1/tools/build/v2/build-system.jam:237: in load /home/ted/boost_1_34_1/libs/python/example/../../../tools/build/v2/kernel/modules.jam:261: in import /home/ted/boost_1_34_1/libs/python/example/../../../tools/build/v2/kernel/bootstrap.jam:132: in boost-build /home/ted/boost_1_34_1/libs/python/example/boost-build.jam:7: in module scope

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How about class_<...>.enable_copy() ?
by Hans Meine Oct. 30, 2007

Oct. 30, 2007

Hi! I often use copy.[deep]copy() to copy python objects / BPL-exported types. This works if pickling support has been explicitly coded, by pickling and unpickling. It would be neat if there was an .enable_copy() or sth. like that, which would define a __copy__ (or __deepcopy__) method that simply calls the copy constructor. That would be more efficient than the pickling approach, both at runtime and in terms of saved LOCs. Have you thought about such a thing? Is there a common short idiom that does that already? Is there a rationale for not providing copying support? -- Ciao, / / /--/ / / ANS

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Re: [C++-sig] stuffing a PyObject into a object
by Ralf W. Grosse-Kunstleve Oct. 30, 2007

Oct. 30, 2007

> > I can understand this except for a couple of things, > > > > What is the PyIter_Next there for? demonstration purposes? or is it an > > essential part of the conversion? Same question for that whole second > > line. > > I think it's a mistake. This is five years ago, but IIRC my reply was meant as food for thought. Why does it have to be a two-stage procedure with the detour through handle<>? My understanding was: it is meant to make you think about PyObject* = 0. > > will 'allow_null' make a 'None' out of a null pointer? or do some > > other crazy thing? > > No, it makes a null handle. It seems superfluous, since the object > ctor will throw an exception anyway if its argument is null. I wanted to give a realistic example why you may care about PyObject* = 0. We want to detect PyObject* = 0 without getting an exception (// end of iteration) *and* we don't want to have the bare PyObject* floating around since we may forget to decref later. > > I am trying to understand what is going on, and not just fall into > > guess and check programming. > > Good call. Guess and check is a fantastic way to find out what is going on (or as Germany's most famous poet, Goethe, put it: Probieren geht ueber studieren). Here is an example: void foo() { using namespace boost::python; PyObject* obj = 0; handle<> h(allow_null(obj)); std::cout << "HELLO" << std::endl << std::flush; } If you compile and run this function it will finish fine and show the HELLO. Without the allow_null() I get this: SystemError: NULL result without error in PyObject_Call and no HELLO. Ralf

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stuffing a PyObject into a object
by Matthew Scouten Oct. 29, 2007

Oct. 29, 2007

Right, so.... I've got a void* that I happen to know is really a PyObject*. (how do i know that you ask? It was handed to me by python and tunneled through a 3rd party c++ library that promised not to mess with it.) I need to make it back into a boost::python::object so I can hand it back to python. I googled for how to do this and came across: http://osdir.com/ml/python.c++/2002-12/msg00286.html where I found the following recommendation: boost::python::handle<> h(boost::python::allow_null(PyIter_Next(x))); if (!h.get()) break; // end of iteration boost::python::object o(h); I can understand this except for a couple of things, What is the PyIter_Next there for? demonstration purposes? or is it an essential part of the conversion? Same question for that whole second line. will 'allow_null' make a 'None' out of a null pointer? or do some other crazy thing? more to the point, will this code work: object foo(void* callbackID == NULL) { boost::python::handle<> h(boost::python::allow_null((PyObject*)callbackID)) return boost::python::object o(h); } I am trying to understand what is going on, and not just fall into guess and check programming.

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Comparing to None on the c++ side
by Matthew Scouten Oct. 29, 2007

Oct. 29, 2007

Right, so... Python just called a c++ function and passed an object of some sort. I want to know if it happens to be a None, so I can take special precautions to avoid disaster. I am trying to use something like: void foo(object o) { if( object() == o ) { //Make 3rdparty c++ lib call that prevents //Sky from falling } else { //tell 3rdparty c++ lib to //try to use o to prop up sky... } } This does not seem to be working like I expected (the sky falls anyway) So me question: is object() == o a valid and reliable way to test for None? If not, how should I be doing it? It is possible that the problem is in 3rdparty c++ lib or my understanding thereof, but I am trying to eliminate the other possibilities.

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Wrapping out parameter for python access
by Chang Chen Oct. 28, 2007

Oct. 28, 2007

Hi all This is a classical issue in boost python. I.e. removeing an argument from the function definition and adding the "returned", by the original function, value to the return statement of the function-wrapper For example : The original function is: inline void hello_world( std::string& hw ){ hw = "hello world!"; } And the wrapper is: inline std::string hello_world_wrapper() { std:string temp; ::hello_world(temp); Return temp; } Py++ can help us to generate such wrapper, see http://language-binding.net/pyplusplus/documentation/functions/transform ation/built_in/output.html But in this thread (http://mail.python.org/pipermail/c++-sig/2005-June/009097.html ), Dave provided a start to write a wrapping template. In My case, only last parameter is out parameter and whether function parameter count does not matter. The function signature looks like: Return function (T1 t1, T2 t2, TOUT& out); How to write a template for such function and integrate into def? Any suggestion? -Chang

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Re: [C++-sig] C++-sig Digest, Vol 51, Issue 26
by Shrikant Chikhalkar Oct. 26, 2007

Oct. 26, 2007

I got solution for the problem. using root credentials run following command on shell prompt $python2 setup.py install where setup.py would be from distutils.core import setup setup(name='xxx', version='1.0', py_modules=['py_function'], ) thanks shrikant On 10/25/07, c++-sig-request(a)python.org <c++-sig-request(a)python.org> wrote: > > Send C++-sig mailing list submissions to > c++-sig(a)python.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.python.org/mailman/listinfo/c++-sig > or, via email, send a message with subject or body 'help' to > c++-sig-request(a)python.org > > You can reach the person managing the list at > c++-sig-owner(a)python.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of C++-sig digest..." > > > Today's Topics: > > 1. Re: C++-sig Digest, Vol 51, Issue 25 (Shrikant Chikhalkar) > 2. Re: C++-sig Digest, Vol 51, Issue 25 (Shrikant Chikhalkar) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 25 Oct 2007 16:33:42 +0530 > From: "Shrikant Chikhalkar" <shrikantvc(a)gmail.com> > Subject: Re: [C++-sig] C++-sig Digest, Vol 51, Issue 25 > To: c++-sig(a)python.org > Message-ID: > <87de5f460710250403q311e8fd1q61de04dc7b935f88(a)mail.gmail.com> > Content-Type: text/plain; charset="iso-8859-1" > > Hi, > thanks for the reply. > that i have already tried. Still i am getting same error. could you please > give some pointer that where i am doing wrong. > > thanks > Shrikant > > > On 10/25/07, c++-sig-request(a)python.org <c++-sig-request(a)python.org> > wrote: > > > > Send C++-sig mailing list submissions to > > c++-sig(a)python.org > > > > To subscribe or unsubscribe via the World Wide Web, visit > > http://mail.python.org/mailman/listinfo/c++-sig > > or, via email, send a message with subject or body 'help' to > > c++-sig-request(a)python.org > > > > You can reach the person managing the list at > > c++-sig-owner(a)python.org > > > > When replying, please edit your Subject line so it is more specific > > than "Re: Contents of C++-sig digest..." > > > > > > Today's Topics: > > > > 1. failed in PyImport_Import() (Shrikant Chikhalkar) > > 2. Re: failed in PyImport_Import() (Gustavo Carneiro) > > > > > > ---------------------------------------------------------------------- > > > > Message: 1 > > Date: Thu, 25 Oct 2007 12:22:24 +0530 > > From: "Shrikant Chikhalkar" <shrikantvc(a)gmail.com> > > Subject: [C++-sig] failed in PyImport_Import() > > To: c++-sig(a)python.org > > Message-ID: > > <87de5f460710242352w5b57dc4djbcfba96ccf0f3578(a)mail.gmail.com> > > Content-Type: text/plain; charset="iso-8859-1" > > > > Hi, > > > > i am tring to call python function from 'C' program. i get NULL value > > return > > by PyImport_Import() > > > > following is the code. code is copied from > > http://www.codeproject.com/cpp/embedpython_1.asp > > > > following file is saved as > > > > call_function.c > > > > > > > /************************************************************************************************************/ > > > > // call_function.c - A sample of calling > > // python functions from C code > > // > > #include <Python.h> > > > > int main(int argc, char *argv[]) > > { > > PyObject *pName, *pModule, *pDict, *pFunc, *pValue; > > > > if (argc < 3) > > { > > printf("Usage: exe_name python_source function_name\n"); > > return 1; > > } > > > > // Initialize the Python Interpreter > > Py_Initialize(); > > > > // Build the name object > > pName = PyString_FromString(argv[1]); > > > > // Load the module object > > pModule = PyImport_Import(pName); > > > > // pDict is a borrowed reference > > pDict = PyModule_GetDict(pModule); > > > > // pFunc is also a borrowed reference > > pFunc = PyDict_GetItemString(pDict, argv[2]); > > > > if (PyCallable_Check(pFunc)) > > { > > PyObject_CallObject(pFunc, NULL); > > } else > > { > > PyErr_Print(); > > } > > > > // Clean up > > Py_DECREF(pModule); > > Py_DECREF(pName); > > > > // Finish the Python Interpreter > > Py_Finalize(); > > > > return 0; > > } > > > > > > > /************************************************************************************************************/ > > python file saved as > > > > py_function.py > > > > > > > /************************************************************************************************************/ > > > > '''py_function.py - Python source designed to ''' > > '''demonstrate the use of python embedding''' > > > > def multiply(): > > c = 12345*6789 > > print 'The result of 12345 x 6789 :', c > > > > > > return c > > > > > > > /************************************************************************************************************/ > > > > $gcc -g call_function.c -lpython2.5 -ocall_function > > $./call_function py_function.py multiply > > > > i get following output > > > > ImportError: No module named py_function > > Failed to load "py_function" > > > > when i kdbg(debug) it i vet NULL value returned from PyImport_Import().. > > > > can any one please find out the error that i am doing. > > > > -- > > Shrikant > >

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