About Python's C API

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    #1

    About Python's C API

    Dear sir,

    I am used to developing C program on MSVC7.0 platform.
    This is my first time to use Python for calling by C program.
    Now, it is hard to deal with the problem about extracting the variable
    which be definied in Python. That how can I do?

    I have gotten the JPython 's sample that the following code can be looked.
    Have anyone help me to obtain same results that be called by Python's C API?
    What do I mean?

    See attached the following code.

    // JPython code
    =============== =============== =========
    import org.python.util .PythonInterpre ter;
    import org.python.core .*;

    public class SimpleEmbedded {
    public static void main(String []args)
    throws PyException
    {
    PythonInterpret er interp =
    new PythonInterpret er();

    System.out.prin tln("Hello, brave new world");
    interp.exec("im port sys");
    interp.exec("pr int sys");

    interp.set("a", new PyInteger(42));
    interp.exec("pr int a");
    interp.exec("x = 2+2");
    PyObject x = interp.get("x") ;

    System.out.prin tln("x: "+x);
    System.out.prin tln("Goodbye, cruel world");
    }
    }


    I would like to re-write those program to Python.

    // C Language code =============== ===============
    void main() {
    Py_Initialize() ;
    printf("Hello, brave new world");

    PyRun_SimpleStr ing("import sys");
    PyRun_SimpleStr ing("print sys");
    // interp.set("a", new PyInteger(42));

    PyRun_SimpleStr ing("print a");
    PyRun_SimpleStr ing("x = 2+2");
    // PyObject x = interp.get("x") ;
    // System.out.prin tln("x: "+x);

    Py_Finalize();
    printf("Goodbye , cruel world");
    }

    See the above C Language code,

    After running the statement PyRun_SimpleStr ing("x = 2+2"),
    the variable x can be definied and be assigned value 4 in Python.

    Meanwhile, next one, I would like to get the x porinter.

    Could you help me how can I do?
    Or show me the other way to reach my purpose.

    Sincerely yours,
    Link


    Tags: py_flushline()
    • Jeff Epler
      #2
      Re: About Python's C API

      Let me start by saying I'm not a pro with the C API of Python.

      With PyRun_SimpleStr ing(), everything takes place in the special
      __main__ module:
      PyRun_SimpleStr ingFlags(const char *command, PyCompilerFlags *flags)
      {
      PyObject *m, *d, *v;
      m = PyImport_AddMod ule("__main__") ;
      if (m == NULL)
      return -1;
      d = PyModule_GetDic t(m);
      v = PyRun_StringFla gs(command, Py_file_input, d, d, flags);
      if (v == NULL) {
      PyErr_Print();
      return -1;
      }
      Py_DECREF(v);
      if (Py_FlushLine() )
      PyErr_Clear();
      return 0;
      }

      So, to get the attribute x from __main__, convert it to a string, and
      printf() it, you'd use this sequence [all untested]:
      PyObject *m, *d, *x, *s;

      /* m is a "new" reference */
      m = PyImport_AddMod ule("__main__") ;
      if (m == NULL)
      return -1;
      /* d is a "borrowed" reference */
      d = PyModule_GetDic t(m);
      Py_DECREF(m); m = NULL; /* Done with m -- decref and set to NULL */
      x = PyDict_GetItemS tring(d, "x")
      /* x is a "borrowed" reference */
      if (x == NULL) {
      PyErr_Print();
      return;
      }
      /* s is a "new" reference */
      s = PyObject_Str(x) ;
      if (s == NULL) {
      PyErr_Print();
      return;
      }
      printf("x: %s\n", PyString_AsStri ng(s));
      Py_DECREF(s); s = NULL; /* Done with s -- decref and set to NULL */

      If you're programming in C++, you might benefit from checking out boost.python at
      Boost.Python
      http://boost.org/libs/python/doc/index.html

      I'm not actually a boost.python user, but I've heard good things about it.

      Jeff

        Comment

        • Link
          #3
          Re: About Python's C API

          Jeff,

          First, I need to say you are so smart.
          I really appreciated your help that of course good idea to solve about my
          extraction's prblem.

          Following your advise, i obtained the value of variable from Python in
          developing C/API.

          By the way, if I had added the statement "Py_DECREF( m); m = NULL;",
          It would be crash during running program.
          So that need to remove this statement.

          Shall I ask you any problem if no this statment at here?
          Any trouble will be happened?

          With best regards.
          Link

          "Jeff Epler" <jepler@unpytho nic.net>
          ???????:mailman .1063850265.961 [email protected] ...[color=blue]
          > Let me start by saying I'm not a pro with the C API of Python.
          >
          > With PyRun_SimpleStr ing(), everything takes place in the special
          > __main__ module:
          > PyRun_SimpleStr ingFlags(const char *command, PyCompilerFlags *flags)
          > {
          > PyObject *m, *d, *v;
          > m = PyImport_AddMod ule("__main__") ;
          > if (m == NULL)
          > return -1;
          > d = PyModule_GetDic t(m);
          > v = PyRun_StringFla gs(command, Py_file_input, d, d, flags);
          > if (v == NULL) {
          > PyErr_Print();
          > return -1;
          > }
          > Py_DECREF(v);
          > if (Py_FlushLine() )
          > PyErr_Clear();
          > return 0;
          > }
          >
          > So, to get the attribute x from __main__, convert it to a string, and
          > printf() it, you'd use this sequence [all untested]:
          > PyObject *m, *d, *x, *s;
          >
          > /* m is a "new" reference */
          > m = PyImport_AddMod ule("__main__") ;
          > if (m == NULL)
          > return -1;
          > /* d is a "borrowed" reference */
          > d = PyModule_GetDic t(m);
          > Py_DECREF(m); m = NULL; /* Done with m -- decref and set to NULL */
          > x = PyDict_GetItemS tring(d, "x")
          > /* x is a "borrowed" reference */
          > if (x == NULL) {
          > PyErr_Print();
          > return;
          > }
          > /* s is a "new" reference */
          > s = PyObject_Str(x) ;
          > if (s == NULL) {
          > PyErr_Print();
          > return;
          > }
          > printf("x: %s\n", PyString_AsStri ng(s));
          > Py_DECREF(s); s = NULL; /* Done with s -- decref and set to NULL */
          >
          > If you're programming in C++, you might benefit from checking out[/color]
          boost.python at[color=blue]
          > http://boost.org/libs/python/doc/index.html
          > I'm not actually a boost.python user, but I've heard good things about it.
          >
          > Jeff
          >[/color]


            Comment

            • Ted Holden
              #4
              Re: About Python's C API

              Link wrote:
              [color=blue]
              > Dear sir,
              >
              > I am used to developing C program on MSVC7.0 platform.
              > This is my first time to use Python for calling by C program.[/color]


              Much easier to use swig than to code that sort of thing directly.

              Simplified Wrapper and Interface Generator
              https://www.swig.org

                Comment

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