Mailman 3 November 2006 - Cplusplus-sig

new call policy - return_pointee_value
by Roman Yakovenko Nov. 8, 2006

Nov. 8, 2006

Good evening. I am working on some library that has a lot functions, that return pointer to some fundamental type. For example: float* get_value(); float* is actually a pointer to some instance of float and not to array. AFAIK, today it is not possible to expose such function without creating small wrapper. I created new call policy - return_pointee_value, that implements next logic: if return value is NULL pointer: return None else: return object( * return value ) Usage example: namespace bpl = boost::python; def( "get_value" , &get_value , bpl::return_value_policy< bpl::return_pointee_value >() ) Source code is attached. I think, this call policy should be added to the library. Thoughts? -- Roman Yakovenko C++ Python language binding http://www.language-binding.net/

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Re: [C++-sig] Insert instance..... again - solved
by Beau Sapach Nov. 8, 2006

Nov. 8, 2006

Hello everyone, Boy I feel sheepish, but I think I've found my problem. In my PyTypeObject I had set tp_basicsize to sizeof(C++_ClassName) instead of the sizeof(Python_struct) where Python_struct is my "typedef struct { PyObject_HEAD ..." that wraps the C++ object. The code I posted below seems to work just fine now. Beau From: Beau Sapach [mailto:beau.sapach@ualberta.ca] Sent: Tuesday, November 07, 2006 3:47 PM To: 'Development of Python/C++ integration' Subject: Insert instance..... again Hello everyone, I know it seems like I'm beating a dead horse here, but I'm not having much luck in understanding the "proper" way to use some of the C API functions. I want to insert an instance of one of my own objects into the python environment, here's what I've got so far. //get the main module PyObject * pMainModule = PyImport_ImportModule("__main__"); //insert MyType into the main module Py_INCREF(MyType); PyModule_AddObject(pMainModule,"MyObject",(PyObject*)&MyType); //create an instance of mytype PyObject * MyInstance = MakeMyObject(); //get the main module's dictionary PyObject * pMainDict = PyModule_GetDict(pMainModule); //insert my instance into the dictionary under the name 'testobject' PyDict_SetItemString(pMainDict,"testobject",MyInstance); //run a script that calls some function of my 'testobject' instance PyRun_SimpleString("testobject.function()\n"); My object implements only a constructor, destructor and one function of its own. The above code crashes somewhere in the python DLL but appears to be trying to access something that's un-initialized. Is there something I'm doing wrong? The above code works fine when I insert a PyObject integer created using PyInt_FromLong(). Minus of course the unnecessary PyModule_AddObject(). What's different about my object? Is there some functionality missing? Should I be implementing some of the other functions in the PyTypeObject? Any help would be appreciated! Beau

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inherit an instance, or something
by Johan Rydberg Nov. 8, 2006

Nov. 8, 2006

I have what might seem to be a strange situation. I have a plugin-system which loads plugins from DSO's. Each plugin presents a factory function which returns an object that implements an abstract interface (a class). The interface could look something like this: class plugin { virtual void set_parameter (const char *name, int value); ... }; The plugin class is easily wrapped using Boost.Python, and a generic factory function could be exported which loads a DSO, finds the factory function, invokes it and returns the created instance; plugin *make_plugin (const char *dso_name); The problem comes when I in python wants to inherit from a specific plugin. The simplest way is probably to do something like this; (case 1) class my_extended_plugin(plugin): def __init__(self, ...): plugin.__init__(self) self.o = make_plugin('libmy-plugin.so') self.set_parameter = self.o.set_parameter But that to me is not a clean solution. It makes it hard to inherit From my_extended_plugin. (not possible to invoke my_extended_plugin.set_parameter correctly.) What I really want is something like this; (case 2, dso_plugin inherits plugin) class my_extended_plugin(dso_plugin): def __init__(self, ...): dso_plugin.__init__(self, 'libmy-plygin.so') Where dso_plugin.__init__ would "replace" the held pointer for 'self' with the one that make_plugin('libmy-plugin.so') returns. Is it possible to do this in anyway, or must I reside to case 1? ~j

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Insert instance..... again
by Beau Sapach Nov. 7, 2006

Nov. 7, 2006

Hello everyone, I know it seems like I'm beating a dead horse here, but I'm not having much luck in understanding the "proper" way to use some of the C API functions. I want to insert an instance of one of my own objects into the python environment, here's what I've got so far. //get the main module PyObject * pMainModule = PyImport_ImportModule("__main__"); //insert MyType into the main module Py_INCREF(MyType); PyModule_AddObject(pMainModule,"MyObject",(PyObject*)&MyType); //create an instance of mytype PyObject * MyInstance = MakeMyObject(); //get the main module's dictionary PyObject * pMainDict = PyModule_GetDict(pMainModule); //insert my instance into the dictionary under the name 'testobject' PyDict_SetItemString(pMainDict,"testobject",MyInstance); //run a script that calls some function of my 'testobject' instance PyRun_SimpleString("testobject.function()\n"); My object implements only a constructor, destructor and one function of its own. The above code crashes somewhere in the python DLL but appears to be trying to access something that's un-initialized. Is there something I'm doing wrong? The above code works fine when I insert a PyObject integer created using PyInt_FromLong(). Minus of course the unnecessary PyModule_AddObject(). What's different about my object? Is there some functionality missing? Should I be implementing some of the other functions in the PyTypeObject? Any help would be appreciated! Beau

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Type conversion problem
by Randall Hopper Nov. 7, 2006

Nov. 7, 2006

I'm trying to update the PyOSG wrappers to allow access to an array type returned from a C++ class method. All attempts to call a Python method on the returned array or it's elements (even print it), result in a Boost.Python.ArgumentError (error msg at bottom). Here's the wrap of the array class (osg::Vec3Array): namespace PyOSG { void init_ArrayVec3() { TemplateCompArray<osg::Vec3Array, osg::Vec3> Vec3Array("Vec3Array"); } } and here's a piece of the referenced template: using namespace boost::python; template <typename T, typename BT> class TemplateCompArray { public : TemplateCompArray(const char * name) : _array(name, boost::python::no_init) { _array .def(init<>()) .def(init<int>()) .def("__init__", make_constructor(&create_from_tuple)) .def("getNumElements", &T::getNumElements) .def("__getitem__", &getitem, return_internal_reference<>()) .def("__setitem__", &setitem, return_internal_reference<>()) .def("resize", &resize) .def("__str__", &str) ... static std::string str(T * self) { ... } } Any idea why printing (or calling a Python method of the wrapped Vec3Array in Python should generate a type error? It's like boost.python doesn't realize the TemplateCompArray instantiation "is" Vec3Array. Thanks, Randall P.S. If you want to see the full source for context, check out: http://mosca.caltech.edu/pyosg_downloads/pyosg_ads_15.tar.gz and look in osg/ArrayVec3.cpp and include/TemplateCompArray.hpp. ------------------------------------------------------------------------------ Traceback (most recent call last): ... File "osg_to_sgg.py", line 147, in setupGeode print geom.getVertexArray() Boost.Python.ArgumentError: Python argument types in Vec3Array.__str__(Vec3Array) did not match C++ signature: __str__(osg::TemplateArray<osg::Vec3f, (osg::Array::Type)10, 3, 5126>*)

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[Boost.python] Converting python file object to std::istream.
by Dan Nov. 6, 2006

Nov. 6, 2006

I am exposing a C++ class to python that takes a reference to a std::istream object and operates on it. It looks something like this: class StreamReader { public: StreamReader(std::istream& strm); private: std::istream& strm_; } I would like to expose this class to python allowing a python client to pass in any python file type object (regular file object, StringIO object, stdin, etc.), as a C++ client would be able to pass any std::istream derived object. Now it seems to me that someone must have encountered this problem before, but I haven't seen any references as to how to make this work. Any ideas on the best way to proceed would be appreciated. thanks, Dan

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Re: [C++-sig] Boost::Python patch for decorated class method
by Richard Berg Nov. 4, 2006

Nov. 4, 2006

Nobody wants to fix this? -----Original Message----- From: Richard Berg [mailto:richardb@avisynth.org] Sent: Thursday, October 26, 2006 1:28 AM To: c++-sig(a)python.org Subject: RE: Boost::Python patch for decorated class method Surprised there was no response from Boost. Is this the right place to submit boost::python bugs & patches? -----Original Message----- From: Richard Berg [mailto:richardb@avisynth.org] Sent: Sunday, October 15, 2006 21:40 To: boost(a)lists.boost.org Subject: Boost::Python patch for decorated class method I found that I could not compile any code that tried to export a class method to Python if the method was "decorated" with a calling convention like __cdecl (MSVC8 style). Standalone functions work ok because the compiler matches them against RT(*) while class methods were trying to match against RT(ClassT::*). I'm sure there's a concise way to make signature.hpp work with decorative keywords generally, so I am *not* proposing you accept my patch. I know virtually nothing about template metaprogramming; my code is neither concise nor general. However, the patch does demonstrate the problem and solution. Best, -Richard

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Re: [C++-sig] Passing an instance...
by Stefan Seefeld Nov. 3, 2006

Nov. 3, 2006

Beau Sapach wrote: > Hello everyone, > > > > I've got a C++ object that I've created a Python type for. When I insert > the object into the main module using PyModule_AddObject then I can > instantiate it in Python and everything works just fine. I assumed (mostly > from reading other's code) that I could use something like: > > > > PyDict_SetItemString(MainDict,"x",pMyObject) Up to this point there is no need to use the C API directly. Assuming you have exported your C++ type 'Type' to python, you can write (inside C++): namespace bpl = boost::python; bpl::object instance(Type(...)); bpl::dict global; global["instance"] = instance; bpl::exec_file(python_script, global, global); and the executed script can access your Type instance directly through the 'instance' variable. Caution: the 'exec_file' function has been added after 1.33.1, so you need to use a CVS snapshot. Regards, Stefan -- ...ich hab' noch einen Koffer in Berlin...

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Passing an instance...
by Beau Sapach Nov. 3, 2006

Nov. 3, 2006

Hello everyone, I've got a C++ object that I've created a Python type for. When I insert the object into the main module using PyModule_AddObject then I can instantiate it in Python and everything works just fine. I assumed (mostly from reading other's code) that I could use something like: PyDict_SetItemString(MainDict,"x",pMyObject) To get a variable called "x" into my python environment that IS the same instance as that pointed to by pMyObject. Then by running PyRun_SimpleString("x.somefunction()\n") I could make use of pMyObject from within Python. This doesn't work, my application crashes. What is the best way to accomplish this using the Py??? functions from the Python API? Beau

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