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List:       python-cpp-sig
Subject:    [C++-sig] Calling a python function from C++.
From:       jeff.holle () verizon ! net (Jeff Holle)
Date:       2004-06-28 15:03:42
Message-ID: 40DF98ED.8070800 () verizon ! net
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I'm using boost 1_31_0 on Mandrake 10.0 Linux.

I've created a simple demonstration of calling a python function with 
C++ object args using python.boost.
It currently works with one exception.  In the face of a missing python 
script, it "aborts" instead of properly throwing an exception if I use 
the "handle<>" machinery that boost.python offers.  The following code 
fragment contains the working code with the stuff that should do the 
equivalent, but exibits this problem, commented out.

PyObject *    /* Returns a borrowed reference */
getPythonFunction(string moduleName,string functionName)
{
//    handle<> _module( PyImport_ImportModule(const_cast<char 
*>(moduleName.c_str())));
//    if ( _module.get() == NULL) throwException("The python module 
\"",moduleName.c_str(),"\" is not available",NULL);
//    handle<> _namespace(borrowed( PyModule_GetDict(_module.get())));
//    if ( _namespace.get() == NULL) throwException("Failed to acquire 
the dictionary of \"",moduleName.c_str(),"\"",NULL);
//    handle<> _function(borrowed(PyDict_GetItemString( 
_namespace.get(),functionName.c_str())));
//    if ( _function.get() == NULL) throwException("Failed to find 
needed function \"",functionName.c_str(),"\"",NULL);
//  return _function.get();
    PyObject *pModule = PyImport_ImportModule(const_cast<char 
*>(moduleName.c_str()));
    if ( pModule == NULL) throwException("The python module 
\"",moduleName.c_str(),"\" is not available",NULL);
    PyObject *pDict = PyModule_GetDict(pModule);
    if ( pDict == NULL) throwException("Failed to acquire the dictionary 
of \"",moduleName.c_str(),"\"",NULL);
    PyObject *pFunc = PyDict_GetItemString(pDict,functionName.c_str());
    if ( pFunc == NULL) throwException("Failed to find requested python 
function \"",functionName.c_str(),"\"",NULL);
    Py_DECREF(pModule);
    return pFunc;
}

In this form, this function throws a std::run_time exception which is 
successfully caught with a "catch (exception& x)" clause.  With the 
commented out section active, something is thrown, but it can only be 
caught with "catch(...)".

I'm wondering if anybody can explain this.

I'm also wondering if I could contribute this example to boost.python.  
The python script that I call is:
    import sys
    from Greeter import *

    def AlterGreeting( greeting):
      greeting.setLanguage("Python")
      return greeting.greet()

    if __name__ == '__main__':
      print 'testing module'
      greetingObj = Greeter("C++")
      greeting = AlterGreeting(greetingObj)
      print greeting

Note that the Greeter module is a C++ extension module also created with 
boost.python using pyste.
The C++ part of this example does the same thing that the main module 
specific section of this python script does.

If anybody wants this example, simple request it.  I'll provide the 
complete project, which uses bjam v1, in a tar ball.


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