How can i use ofstream in class that is wrapped with Boost.Python to Python?

Question

I changed the code one month ago and im stucking at the same error which i described below. I dont find a pretty simple example how to expose an fstream object with Boost.Python to Python to solve my explained problem

In short words i just want to expose a Class that contains an I/O object with functions write / open / close. In Pyton i want to do these steps:

1. open File1
2. open File2
3. open File3
4. write to File1
5. write to File2
6. write to File3
7. close files

C/C++ Code

////// I N C L U D E S //////
#include <boost/python/module.hpp>
#include <boost/python/def.hpp>
#include <boost/python.hpp>
#include <fstream>
#include <string.h>
////////////////////////////


using namespace std;

struct BoostXML_IO
{
    ofstream File;

    void writeFile(const string& strToWrite)
    {
        this->File.write(strToWrite.data(), strToWrite.size());
    }

    void openFile(const string& path)
    {
        this->File.open(path, ios::app);
    }

    void closeFile()
    {
        this->File.close();
    }

};


BOOST_PYTHON_MODULE(BoostXML_IO)
{
    using namespace boost::python;  
    class_<BoostXML_IO, boost::noncopyable>("BoostXML_IO")
      .def("writeFile", &BoostXML_IO::writeFile)
      .def("openFile", &BoostXML_IO::openFile)
      .def("closeFile", &BoostXML_IO::closeFile)
    ;
}

This code always compiles without an error but finally in Python i always get the following error when i try to invoke one of the functions in the prompt line.

ERROR code

>>> import BoostXML_IO
>>> File1 = BoostXML_IO
>>> File = BoostXML_IO.BoostXML_IO.openFileFailed to format the args
Traceback (most recent call last):
  File "C:\app\tools\Python25\Lib\site-packages\pythonwin\pywin\idle\CallTips.py", line 130, in get_arg_text
    argText = inspect.formatargspec(*arg_getter(fob))
  File "C:\app\tools\Python25\Lib\inspect.py", line 743, in getargspec
    raise TypeError('arg is not a Python function')
TypeError: arg is not a Python function
(

Thank you a lot in advance !

Answer 1

Your compile errors have nothing to do with Boost.Python. Start by including <string> and <ofstream> and add a using namespace std;. That should fix most of the errors you're getting. Then fix your struct declaration like so:

struct BoostXML_IO
{    
  void openFile(const string& path)
  {
    myfile.open(path);
  }
  void closeFile()
  {
    myfile.close();
  }
  void WriteToFile(const char* strToWrite)
  {
    myfile.write(strToWrite, strlen(strToWrite));
  }

private:
  ofstream myFile;
};

I also don't see a need for setting a readwrite reference to the strToWrite and path members when you pass them as parameters in openFile and WriteToFile.

---

EDIT What about this:

struct BoostXML_IO
{
    BoostXML_IO()
    {}

    void writeFile(const string& strToWrite)
    {
        File.write(strToWrite.data(), strToWrite.size());
    }

    void openFile(const string& path)
    {
        File.open(path, ios::out);
    }

    void closeFile()
    {
        File.close();
    }

private:
    BoostXML_IO(const BoostXML_IO&);
    BoostXML_IO& operator=(const BoostXML_IO&);

    ofstream File;
};


BOOST_PYTHON_MODULE(BoostXML_IO)
{
    using namespace boost::python;  
    class_<BoostXML_IO>("BoostXML_IO", init<>())
      .def("writeFile", &BoostXML_IO::writeFile)
      .def("openFile", &BoostXML_IO::openFile)
      .def("closeFile", &BoostXML_IO::closeFile)
    ;
}

I have no idea why you'd want to expose an ofstream to the python API. Same for the pathToFile variable which is kind of useless when specifying the file path in openFile. But I think I mentioned that already.