symbian-qemu-0.9.1-12/python-2.6.1/Doc/extending/embedding.rst
author martin.trojer@nokia.com
Fri, 31 Jul 2009 15:01:17 +0100
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.. highlightlang:: c
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.. _embedding:
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Embedding Python in Another Application
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***************************************
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The previous chapters discussed how to extend Python, that is, how to extend the
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functionality of Python by attaching a library of C functions to it.  It is also
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possible to do it the other way around: enrich your C/C++ application by
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embedding Python in it.  Embedding provides your application with the ability to
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implement some of the functionality of your application in Python rather than C
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or C++. This can be used for many purposes; one example would be to allow users
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to tailor the application to their needs by writing some scripts in Python.  You
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can also use it yourself if some of the functionality can be written in Python
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more easily.
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Embedding Python is similar to extending it, but not quite.  The difference is
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that when you extend Python, the main program of the application is still the
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Python interpreter, while if you embed Python, the main program may have nothing
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to do with Python --- instead, some parts of the application occasionally call
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the Python interpreter to run some Python code.
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So if you are embedding Python, you are providing your own main program.  One of
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the things this main program has to do is initialize the Python interpreter.  At
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the very least, you have to call the function :cfunc:`Py_Initialize`.  There are
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optional calls to pass command line arguments to Python.  Then later you can
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call the interpreter from any part of the application.
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There are several different ways to call the interpreter: you can pass a string
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containing Python statements to :cfunc:`PyRun_SimpleString`, or you can pass a
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stdio file pointer and a file name (for identification in error messages only)
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to :cfunc:`PyRun_SimpleFile`.  You can also call the lower-level operations
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described in the previous chapters to construct and use Python objects.
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A simple demo of embedding Python can be found in the directory
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:file:`Demo/embed/` of the source distribution.
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.. seealso::
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   :ref:`c-api-index`
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      The details of Python's C interface are given in this manual. A great deal of
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      necessary information can be found here.
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.. _high-level-embedding:
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Very High Level Embedding
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=========================
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The simplest form of embedding Python is the use of the very high level
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interface. This interface is intended to execute a Python script without needing
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to interact with the application directly. This can for example be used to
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perform some operation on a file. ::
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   #include <Python.h>
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   int
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   main(int argc, char *argv[])
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   {
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     Py_Initialize();
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     PyRun_SimpleString("from time import time,ctime\n"
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                        "print 'Today is',ctime(time())\n");
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     Py_Finalize();
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     return 0;
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   }
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The above code first initializes the Python interpreter with
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:cfunc:`Py_Initialize`, followed by the execution of a hard-coded Python script
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that print the date and time.  Afterwards, the :cfunc:`Py_Finalize` call shuts
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the interpreter down, followed by the end of the program.  In a real program,
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you may want to get the Python script from another source, perhaps a text-editor
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routine, a file, or a database.  Getting the Python code from a file can better
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be done by using the :cfunc:`PyRun_SimpleFile` function, which saves you the
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trouble of allocating memory space and loading the file contents.
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.. _lower-level-embedding:
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Beyond Very High Level Embedding: An overview
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=============================================
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The high level interface gives you the ability to execute arbitrary pieces of
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Python code from your application, but exchanging data values is quite
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cumbersome to say the least. If you want that, you should use lower level calls.
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At the cost of having to write more C code, you can achieve almost anything.
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It should be noted that extending Python and embedding Python is quite the same
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activity, despite the different intent. Most topics discussed in the previous
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chapters are still valid. To show this, consider what the extension code from
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Python to C really does:
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#. Convert data values from Python to C,
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#. Perform a function call to a C routine using the converted values, and
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#. Convert the data values from the call from C to Python.
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When embedding Python, the interface code does:
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#. Convert data values from C to Python,
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#. Perform a function call to a Python interface routine using the converted
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   values, and
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#. Convert the data values from the call from Python to C.
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As you can see, the data conversion steps are simply swapped to accommodate the
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different direction of the cross-language transfer. The only difference is the
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routine that you call between both data conversions. When extending, you call a
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C routine, when embedding, you call a Python routine.
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This chapter will not discuss how to convert data from Python to C and vice
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versa.  Also, proper use of references and dealing with errors is assumed to be
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understood.  Since these aspects do not differ from extending the interpreter,
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you can refer to earlier chapters for the required information.
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.. _pure-embedding:
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Pure Embedding
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==============
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The first program aims to execute a function in a Python script. Like in the
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section about the very high level interface, the Python interpreter does not
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directly interact with the application (but that will change in the next
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section).
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The code to run a function defined in a Python script is:
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.. literalinclude:: ../includes/run-func.c
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This code loads a Python script using ``argv[1]``, and calls the function named
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in ``argv[2]``.  Its integer arguments are the other values of the ``argv``
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array.  If you compile and link this program (let's call the finished executable
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:program:`call`), and use it to execute a Python script, such as::
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   def multiply(a,b):
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       print "Will compute", a, "times", b
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       c = 0
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       for i in range(0, a):
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           c = c + b
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       return c
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then the result should be::
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   $ call multiply multiply 3 2
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   Will compute 3 times 2
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   Result of call: 6
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Although the program is quite large for its functionality, most of the code is
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for data conversion between Python and C, and for error reporting.  The
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interesting part with respect to embedding Python starts with ::
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   Py_Initialize();
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   pName = PyString_FromString(argv[1]);
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   /* Error checking of pName left out */
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   pModule = PyImport_Import(pName);
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After initializing the interpreter, the script is loaded using
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:cfunc:`PyImport_Import`.  This routine needs a Python string as its argument,
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which is constructed using the :cfunc:`PyString_FromString` data conversion
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routine. ::
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   pFunc = PyObject_GetAttrString(pModule, argv[2]);
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   /* pFunc is a new reference */
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   if (pFunc && PyCallable_Check(pFunc)) {
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       ...
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   }
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   Py_XDECREF(pFunc);
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Once the script is loaded, the name we're looking for is retrieved using
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:cfunc:`PyObject_GetAttrString`.  If the name exists, and the object returned is
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callable, you can safely assume that it is a function.  The program then
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proceeds by constructing a tuple of arguments as normal.  The call to the Python
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function is then made with::
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   pValue = PyObject_CallObject(pFunc, pArgs);
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Upon return of the function, ``pValue`` is either *NULL* or it contains a
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reference to the return value of the function.  Be sure to release the reference
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after examining the value.
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.. _extending-with-embedding:
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Extending Embedded Python
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=========================
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Until now, the embedded Python interpreter had no access to functionality from
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the application itself.  The Python API allows this by extending the embedded
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interpreter.  That is, the embedded interpreter gets extended with routines
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provided by the application. While it sounds complex, it is not so bad.  Simply
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forget for a while that the application starts the Python interpreter.  Instead,
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consider the application to be a set of subroutines, and write some glue code
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that gives Python access to those routines, just like you would write a normal
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Python extension.  For example::
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   static int numargs=0;
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   /* Return the number of arguments of the application command line */
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   static PyObject*
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   emb_numargs(PyObject *self, PyObject *args)
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   {
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       if(!PyArg_ParseTuple(args, ":numargs"))
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           return NULL;
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       return Py_BuildValue("i", numargs);
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   }
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   static PyMethodDef EmbMethods[] = {
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       {"numargs", emb_numargs, METH_VARARGS,
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        "Return the number of arguments received by the process."},
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       {NULL, NULL, 0, NULL}
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   };
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Insert the above code just above the :cfunc:`main` function. Also, insert the
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following two statements directly after :cfunc:`Py_Initialize`::
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   numargs = argc;
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   Py_InitModule("emb", EmbMethods);
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These two lines initialize the ``numargs`` variable, and make the
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:func:`emb.numargs` function accessible to the embedded Python interpreter.
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With these extensions, the Python script can do things like ::
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   import emb
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   print "Number of arguments", emb.numargs()
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In a real application, the methods will expose an API of the application to
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Python.
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.. TODO: threads, code examples do not really behave well if errors happen
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   (what to watch out for)
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.. _embeddingincplusplus:
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Embedding Python in C++
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=======================
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It is also possible to embed Python in a C++ program; precisely how this is done
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will depend on the details of the C++ system used; in general you will need to
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write the main program in C++, and use the C++ compiler to compile and link your
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program.  There is no need to recompile Python itself using C++.
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.. _link-reqs:
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Linking Requirements
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====================
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While the :program:`configure` script shipped with the Python sources will
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correctly build Python to export the symbols needed by dynamically linked
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extensions, this is not automatically inherited by applications which embed the
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Python library statically, at least on Unix.  This is an issue when the
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application is linked to the static runtime library (:file:`libpython.a`) and
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needs to load dynamic extensions (implemented as :file:`.so` files).
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The problem is that some entry points are defined by the Python runtime solely
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for extension modules to use.  If the embedding application does not use any of
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these entry points, some linkers will not include those entries in the symbol
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table of the finished executable.  Some additional options are needed to inform
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the linker not to remove these symbols.
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Determining the right options to use for any given platform can be quite
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difficult, but fortunately the Python configuration already has those values.
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To retrieve them from an installed Python interpreter, start an interactive
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interpreter and have a short session like this::
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   >>> import distutils.sysconfig
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   >>> distutils.sysconfig.get_config_var('LINKFORSHARED')
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   '-Xlinker -export-dynamic'
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.. index:: module: distutils.sysconfig
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The contents of the string presented will be the options that should be used.
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If the string is empty, there's no need to add any additional options.  The
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:const:`LINKFORSHARED` definition corresponds to the variable of the same name
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in Python's top-level :file:`Makefile`.
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