Liwei Peng <lpeng@[...].edu>  writes:

>  Hi,
>  
>  I am using Boost Python V2 to wrap my C++ code. I failed
>  to convert python file to/from C++ FILE*. Here is my code:
>  
>  ----------------------------------------
>  struct FILE_to_pyfile
>  {
>      static PyObject* convert(FILE* x)
>      {        
>          return PyFile_FromFile(x, "", "", NULL);
>      }
>  };
>  
>  FILE* getfile(const char* fname)
>  {
>      printf("openning file: %s\n", fname);
>      FILE* f = fopen(fname, "w");
>      fprintf(f, "initial words\n");
>      return f;
>  }
>  
>  BOOST_PYTHON_MODULE(numbermod)
>  {
>      python::to_python_converter<FILE*, FILE_to_pyfile>();
>      python::def("getfile", getfile,
>                  python::return_internal_reference<>());
>  }
>  
>  -----------------------------------------------
>  After I compiled the code, in python run
>  
>    f = getfile("hello.txt")
>    
>  gave me the following error:
>  
>  Traceback (most recent call last):
>    File "./test.py", line 18, in ?
>      f1 = getfile("hello.txt")
>  TypeError: bad argument type for built-in operation
>  
>  
>  Can you kindly tell me
>    1) what's wrong with the above code? 

The problem is that when converting a pointer to python, Boost.Python
looks up a converter for the _pointee_. So you need to register a
to_python converter for FILE, not FILE*. This should probably be a lot
clearer in my documentation than it is right now.

So I suggest as a first step:

    struct FILE_to_pyfile
    {
        static PyObject* convert(FILE const& x)
        {        
            return PyFile_FromFile(&x, "", "", NULL);
        }
    };

    FILE* getfile(const char* fname)
    {
        printf("openning file: %s\n", fname);
        FILE* f = fopen(fname, "w");
        fprintf(f, "initial words\n");
        return f;
    }

    BOOST_PYTHON_MODULE(numbermod)
    {
        python::to_python_converter<FILE, FILE_to_pyfile> ();
        python::def("getfile", getfile,
                    python::return_value_policy<reference_existing_object> ());
    }

However, this is really not the best arrangement, since it shouldn't
be neccessary to use return_value_policy<reference_existing_object> 
for FILE*. Probably we should add converter specializations for FILE*
just as we have got for the builtin types.

>    2) The above code is from C++ 'FILE*' to python file.
>       If I want to convert python file to C++ file,
>       how can I do that?

You need to register an lvalue converter for FILE. Something like:

    struct pyfile_to_FILE
    {
        static File& execute(PyObject& o) { return *PyFile_AsFile(&o); }
    }

    // In your module init function
    lvalue_from_pytype<pyfile_to_FILE,&PyFile_Type> ();

But again, I think these should probably be done by the library,
through specialization rather than dynamically registered converters.

HTH,
-- 
                    David Abrahams
dave@[...].com * http://www.boost-consulting.com


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