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+++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/extending/embedding.rst	Fri Jul 31 15:01:17 2009 +0100
@@ -0,0 +1,285 @@
+.. highlightlang:: c
+
+
+.. _embedding:
+
+***************************************
+Embedding Python in Another Application
+***************************************
+
+The previous chapters discussed how to extend Python, that is, how to extend the
+functionality of Python by attaching a library of C functions to it.  It is also
+possible to do it the other way around: enrich your C/C++ application by
+embedding Python in it.  Embedding provides your application with the ability to
+implement some of the functionality of your application in Python rather than C
+or C++. This can be used for many purposes; one example would be to allow users
+to tailor the application to their needs by writing some scripts in Python.  You
+can also use it yourself if some of the functionality can be written in Python
+more easily.
+
+Embedding Python is similar to extending it, but not quite.  The difference is
+that when you extend Python, the main program of the application is still the
+Python interpreter, while if you embed Python, the main program may have nothing
+to do with Python --- instead, some parts of the application occasionally call
+the Python interpreter to run some Python code.
+
+So if you are embedding Python, you are providing your own main program.  One of
+the things this main program has to do is initialize the Python interpreter.  At
+the very least, you have to call the function :cfunc:`Py_Initialize`.  There are
+optional calls to pass command line arguments to Python.  Then later you can
+call the interpreter from any part of the application.
+
+There are several different ways to call the interpreter: you can pass a string
+containing Python statements to :cfunc:`PyRun_SimpleString`, or you can pass a
+stdio file pointer and a file name (for identification in error messages only)
+to :cfunc:`PyRun_SimpleFile`.  You can also call the lower-level operations
+described in the previous chapters to construct and use Python objects.
+
+A simple demo of embedding Python can be found in the directory
+:file:`Demo/embed/` of the source distribution.
+
+
+.. seealso::
+
+   :ref:`c-api-index`
+      The details of Python's C interface are given in this manual. A great deal of
+      necessary information can be found here.
+
+
+.. _high-level-embedding:
+
+Very High Level Embedding
+=========================
+
+The simplest form of embedding Python is the use of the very high level
+interface. This interface is intended to execute a Python script without needing
+to interact with the application directly. This can for example be used to
+perform some operation on a file. ::
+
+   #include <Python.h>
+
+   int
+   main(int argc, char *argv[])
+   {
+     Py_Initialize();
+     PyRun_SimpleString("from time import time,ctime\n"
+                        "print 'Today is',ctime(time())\n");
+     Py_Finalize();
+     return 0;
+   }
+
+The above code first initializes the Python interpreter with
+:cfunc:`Py_Initialize`, followed by the execution of a hard-coded Python script
+that print the date and time.  Afterwards, the :cfunc:`Py_Finalize` call shuts
+the interpreter down, followed by the end of the program.  In a real program,
+you may want to get the Python script from another source, perhaps a text-editor
+routine, a file, or a database.  Getting the Python code from a file can better
+be done by using the :cfunc:`PyRun_SimpleFile` function, which saves you the
+trouble of allocating memory space and loading the file contents.
+
+
+.. _lower-level-embedding:
+
+Beyond Very High Level Embedding: An overview
+=============================================
+
+The high level interface gives you the ability to execute arbitrary pieces of
+Python code from your application, but exchanging data values is quite
+cumbersome to say the least. If you want that, you should use lower level calls.
+At the cost of having to write more C code, you can achieve almost anything.
+
+It should be noted that extending Python and embedding Python is quite the same
+activity, despite the different intent. Most topics discussed in the previous
+chapters are still valid. To show this, consider what the extension code from
+Python to C really does:
+
+#. Convert data values from Python to C,
+
+#. Perform a function call to a C routine using the converted values, and
+
+#. Convert the data values from the call from C to Python.
+
+When embedding Python, the interface code does:
+
+#. Convert data values from C to Python,
+
+#. Perform a function call to a Python interface routine using the converted
+   values, and
+
+#. Convert the data values from the call from Python to C.
+
+As you can see, the data conversion steps are simply swapped to accommodate the
+different direction of the cross-language transfer. The only difference is the
+routine that you call between both data conversions. When extending, you call a
+C routine, when embedding, you call a Python routine.
+
+This chapter will not discuss how to convert data from Python to C and vice
+versa.  Also, proper use of references and dealing with errors is assumed to be
+understood.  Since these aspects do not differ from extending the interpreter,
+you can refer to earlier chapters for the required information.
+
+
+.. _pure-embedding:
+
+Pure Embedding
+==============
+
+The first program aims to execute a function in a Python script. Like in the
+section about the very high level interface, the Python interpreter does not
+directly interact with the application (but that will change in the next
+section).
+
+The code to run a function defined in a Python script is:
+
+.. literalinclude:: ../includes/run-func.c
+
+
+This code loads a Python script using ``argv[1]``, and calls the function named
+in ``argv[2]``.  Its integer arguments are the other values of the ``argv``
+array.  If you compile and link this program (let's call the finished executable
+:program:`call`), and use it to execute a Python script, such as::
+
+   def multiply(a,b):
+       print "Will compute", a, "times", b
+       c = 0
+       for i in range(0, a):
+           c = c + b
+       return c
+
+then the result should be::
+
+   $ call multiply multiply 3 2
+   Will compute 3 times 2
+   Result of call: 6
+
+Although the program is quite large for its functionality, most of the code is
+for data conversion between Python and C, and for error reporting.  The
+interesting part with respect to embedding Python starts with ::
+
+   Py_Initialize();
+   pName = PyString_FromString(argv[1]);
+   /* Error checking of pName left out */
+   pModule = PyImport_Import(pName);
+
+After initializing the interpreter, the script is loaded using
+:cfunc:`PyImport_Import`.  This routine needs a Python string as its argument,
+which is constructed using the :cfunc:`PyString_FromString` data conversion
+routine. ::
+
+   pFunc = PyObject_GetAttrString(pModule, argv[2]);
+   /* pFunc is a new reference */
+
+   if (pFunc && PyCallable_Check(pFunc)) {
+       ...
+   }
+   Py_XDECREF(pFunc);
+
+Once the script is loaded, the name we're looking for is retrieved using
+:cfunc:`PyObject_GetAttrString`.  If the name exists, and the object returned is
+callable, you can safely assume that it is a function.  The program then
+proceeds by constructing a tuple of arguments as normal.  The call to the Python
+function is then made with::
+
+   pValue = PyObject_CallObject(pFunc, pArgs);
+
+Upon return of the function, ``pValue`` is either *NULL* or it contains a
+reference to the return value of the function.  Be sure to release the reference
+after examining the value.
+
+
+.. _extending-with-embedding:
+
+Extending Embedded Python
+=========================
+
+Until now, the embedded Python interpreter had no access to functionality from
+the application itself.  The Python API allows this by extending the embedded
+interpreter.  That is, the embedded interpreter gets extended with routines
+provided by the application. While it sounds complex, it is not so bad.  Simply
+forget for a while that the application starts the Python interpreter.  Instead,
+consider the application to be a set of subroutines, and write some glue code
+that gives Python access to those routines, just like you would write a normal
+Python extension.  For example::
+
+   static int numargs=0;
+
+   /* Return the number of arguments of the application command line */
+   static PyObject*
+   emb_numargs(PyObject *self, PyObject *args)
+   {
+       if(!PyArg_ParseTuple(args, ":numargs"))
+           return NULL;
+       return Py_BuildValue("i", numargs);
+   }
+
+   static PyMethodDef EmbMethods[] = {
+       {"numargs", emb_numargs, METH_VARARGS,
+        "Return the number of arguments received by the process."},
+       {NULL, NULL, 0, NULL}
+   };
+
+Insert the above code just above the :cfunc:`main` function. Also, insert the
+following two statements directly after :cfunc:`Py_Initialize`::
+
+   numargs = argc;
+   Py_InitModule("emb", EmbMethods);
+
+These two lines initialize the ``numargs`` variable, and make the
+:func:`emb.numargs` function accessible to the embedded Python interpreter.
+With these extensions, the Python script can do things like ::
+
+   import emb
+   print "Number of arguments", emb.numargs()
+
+In a real application, the methods will expose an API of the application to
+Python.
+
+.. TODO: threads, code examples do not really behave well if errors happen
+   (what to watch out for)
+
+
+.. _embeddingincplusplus:
+
+Embedding Python in C++
+=======================
+
+It is also possible to embed Python in a C++ program; precisely how this is done
+will depend on the details of the C++ system used; in general you will need to
+write the main program in C++, and use the C++ compiler to compile and link your
+program.  There is no need to recompile Python itself using C++.
+
+
+.. _link-reqs:
+
+Linking Requirements
+====================
+
+While the :program:`configure` script shipped with the Python sources will
+correctly build Python to export the symbols needed by dynamically linked
+extensions, this is not automatically inherited by applications which embed the
+Python library statically, at least on Unix.  This is an issue when the
+application is linked to the static runtime library (:file:`libpython.a`) and
+needs to load dynamic extensions (implemented as :file:`.so` files).
+
+The problem is that some entry points are defined by the Python runtime solely
+for extension modules to use.  If the embedding application does not use any of
+these entry points, some linkers will not include those entries in the symbol
+table of the finished executable.  Some additional options are needed to inform
+the linker not to remove these symbols.
+
+Determining the right options to use for any given platform can be quite
+difficult, but fortunately the Python configuration already has those values.
+To retrieve them from an installed Python interpreter, start an interactive
+interpreter and have a short session like this::
+
+   >>> import distutils.sysconfig
+   >>> distutils.sysconfig.get_config_var('LINKFORSHARED')
+   '-Xlinker -export-dynamic'
+
+.. index:: module: distutils.sysconfig
+
+The contents of the string presented will be the options that should be used.
+If the string is empty, there's no need to add any additional options.  The
+:const:`LINKFORSHARED` definition corresponds to the variable of the same name
+in Python's top-level :file:`Makefile`.
+