Doc/faq/extending.rst - platform/external/python/cpython3 - Gitiles

 =======================
 Extending/Embedding FAQ
 =======================

 .. contents::

 .. highlight:: c


 .. XXX need review for Python 3.


 Can I create my own functions in C?
 -----------------------------------

 Yes, you can create built-in modules containing functions, variables, exceptions
 and even new types in C.  This is explained in the document
 :ref:`extending-index`.

 Most intermediate or advanced Python books will also cover this topic.


 Can I create my own functions in C++?
 -------------------------------------

 Yes, using the C compatibility features found in C++.  Place ``extern "C" {
 ... }`` around the Python include files and put ``extern "C"`` before each
 function that is going to be called by the Python interpreter.  Global or static
 C++ objects with constructors are probably not a good idea.


 .. _c-wrapper-software:

 Writing C is hard; are there any alternatives?
 ----------------------------------------------

 There are a number of alternatives to writing your own C extensions, depending
 on what you're trying to do.

 .. XXX make sure these all work; mention Cython

 If you need more speed, `Psyco <http://psyco.sourceforge.net/>`_ generates x86
 assembly code from Python bytecode.  You can use Psyco to compile the most
 time-critical functions in your code, and gain a significant improvement with
 very little effort, as long as you're running on a machine with an
 x86-compatible processor.

 `Pyrex <http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/>`_ is a compiler
 that accepts a slightly modified form of Python and generates the corresponding
 C code.  Pyrex makes it possible to write an extension without having to learn
 Python's C API.

 If you need to interface to some C or C++ library for which no Python extension
 currently exists, you can try wrapping the library's data types and functions
 with a tool such as `SWIG <http://www.swig.org>`_.  `SIP
 <http://www.riverbankcomputing.co.uk/software/sip/>`__, `CXX
 <http://cxx.sourceforge.net/>`_ `Boost
 <http://www.boost.org/libs/python/doc/index.html>`_, or `Weave
 <http://www.scipy.org/Weave>`_ are also alternatives for wrapping C++ libraries.


 How can I execute arbitrary Python statements from C?
 -----------------------------------------------------

 The highest-level function to do this is :cfunc:`PyRun_SimpleString` which takes
 a single string argument to be executed in the context of the module
 ``__main__`` and returns 0 for success and -1 when an exception occurred
 (including ``SyntaxError``).  If you want more control, use
 :cfunc:`PyRun_String`; see the source for :cfunc:`PyRun_SimpleString` in
 ``Python/pythonrun.c``.


 How can I evaluate an arbitrary Python expression from C?
 ---------------------------------------------------------

 Call the function :cfunc:`PyRun_String` from the previous question with the
 start symbol :cdata:`Py_eval_input`; it parses an expression, evaluates it and
 returns its value.


 How do I extract C values from a Python object?
 -----------------------------------------------

 That depends on the object's type.  If it's a tuple, :cfunc:`PyTuple_Size`
 returns its length and :cfunc:`PyTuple_GetItem` returns the item at a specified
 index.  Lists have similar functions, :cfunc:`PyListSize` and
 :cfunc:`PyList_GetItem`.

 For strings, :cfunc:`PyString_Size` returns its length and
 :cfunc:`PyString_AsString` a pointer to its value.  Note that Python strings may
 contain null bytes so C's :cfunc:`strlen` should not be used.

 To test the type of an object, first make sure it isn't *NULL*, and then use
 :cfunc:`PyString_Check`, :cfunc:`PyTuple_Check`, :cfunc:`PyList_Check`, etc.

 There is also a high-level API to Python objects which is provided by the
 so-called 'abstract' interface -- read ``Include/abstract.h`` for further
 details.  It allows interfacing with any kind of Python sequence using calls
 like :cfunc:`PySequence_Length`, :cfunc:`PySequence_GetItem`, etc.)  as well as
 many other useful protocols.


 How do I use Py_BuildValue() to create a tuple of arbitrary length?
 -------------------------------------------------------------------

 You can't.  Use ``t = PyTuple_New(n)`` instead, and fill it with objects using
 ``PyTuple_SetItem(t, i, o)`` -- note that this "eats" a reference count of
 ``o``, so you have to :cfunc:`Py_INCREF` it.  Lists have similar functions
 ``PyList_New(n)`` and ``PyList_SetItem(l, i, o)``.  Note that you *must* set all
 the tuple items to some value before you pass the tuple to Python code --
 ``PyTuple_New(n)`` initializes them to NULL, which isn't a valid Python value.


 How do I call an object's method from C?
 ----------------------------------------

 The :cfunc:`PyObject_CallMethod` function can be used to call an arbitrary
 method of an object.  The parameters are the object, the name of the method to
 call, a format string like that used with :cfunc:`Py_BuildValue`, and the
 argument values::

    PyObject *
    PyObject_CallMethod(PyObject *object, char *method_name,
                        char *arg_format, ...);

 This works for any object that has methods -- whether built-in or user-defined.
 You are responsible for eventually :cfunc:`Py_DECREF`\ 'ing the return value.

 To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the
 file object pointer is "f")::

    res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
    if (res == NULL) {
            ... an exception occurred ...
    }
    else {
            Py_DECREF(res);
    }

 Note that since :cfunc:`PyObject_CallObject` *always* wants a tuple for the
 argument list, to call a function without arguments, pass "()" for the format,
 and to call a function with one argument, surround the argument in parentheses,
 e.g. "(i)".


 How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?
 ----------------------------------------------------------------------------------------

 In Python code, define an object that supports the ``write()`` method.  Assign
 this object to :data:`sys.stdout` and :data:`sys.stderr`.  Call print_error, or
 just allow the standard traceback mechanism to work. Then, the output will go
 wherever your ``write()`` method sends it.

 The easiest way to do this is to use the StringIO class in the standard library.

 Sample code and use for catching stdout:

    >>> class StdoutCatcher:
    ...     def __init__(self):
    ...         self.data = ''
    ...     def write(self, stuff):
    ...         self.data = self.data + stuff
    ...
    >>> import sys
    >>> sys.stdout = StdoutCatcher()
    >>> print('foo')
    >>> print('hello world!')
    >>> sys.stderr.write(sys.stdout.data)
    foo
    hello world!


 How do I access a module written in Python from C?
 --------------------------------------------------

 You can get a pointer to the module object as follows::

    module = PyImport_ImportModule("<modulename>");

 If the module hasn't been imported yet (i.e. it is not yet present in
 :data:`sys.modules`), this initializes the module; otherwise it simply returns
 the value of ``sys.modules["<modulename>"]``.  Note that it doesn't enter the
 module into any namespace -- it only ensures it has been initialized and is
 stored in :data:`sys.modules`.

 You can then access the module's attributes (i.e. any name defined in the
 module) as follows::

    attr = PyObject_GetAttrString(module, "<attrname>");

 Calling :cfunc:`PyObject_SetAttrString` to assign to variables in the module
 also works.


 How do I interface to C++ objects from Python?
 ----------------------------------------------

 Depending on your requirements, there are many approaches.  To do this manually,
 begin by reading :ref:`the "Extending and Embedding" document
 <extending-index>`.  Realize that for the Python run-time system, there isn't a
 whole lot of difference between C and C++ -- so the strategy of building a new
 Python type around a C structure (pointer) type will also work for C++ objects.

 For C++ libraries, you can look at `SIP
 <http://www.riverbankcomputing.co.uk/sip/>`_, `CXX
 <http://cxx.sourceforge.net/>`_, `Boost
 <http://www.boost.org/libs/python/doc/index.html>`_, `Weave
 <http://www.scipy.org/Weave>`_ or `SWIG <http://www.swig.org>`_


 I added a module using the Setup file and the make fails; why?
 --------------------------------------------------------------

 Setup must end in a newline, if there is no newline there, the build process
 fails.  (Fixing this requires some ugly shell script hackery, and this bug is so
 minor that it doesn't seem worth the effort.)


 How do I debug an extension?
 ----------------------------

 When using GDB with dynamically loaded extensions, you can't set a breakpoint in
 your extension until your extension is loaded.

 In your ``.gdbinit`` file (or interactively), add the command::

    br _PyImport_LoadDynamicModule

 Then, when you run GDB::

    $ gdb /local/bin/python
    gdb) run myscript.py
    gdb) continue # repeat until your extension is loaded
    gdb) finish   # so that your extension is loaded
    gdb) br myfunction.c:50
    gdb) continue

 I want to compile a Python module on my Linux system, but some files are missing. Why?
 --------------------------------------------------------------------------------------

 Most packaged versions of Python don't include the
 :file:`/usr/lib/python2.{x}/config/` directory, which contains various files
 required for compiling Python extensions.

 For Red Hat, install the python-devel RPM to get the necessary files.

 For Debian, run ``apt-get install python-dev``.


 What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean?
 -------------------------------------------------------------------------------------

 This means that you have created an extension module named "yourmodule", but
 your module init function does not initialize with that name.

 Every module init function will have a line similar to::

    module = Py_InitModule("yourmodule", yourmodule_functions);

 If the string passed to this function is not the same name as your extension
 module, the :exc:`SystemError` exception will be raised.


 How do I tell "incomplete input" from "invalid input"?
 ------------------------------------------------------

 Sometimes you want to emulate the Python interactive interpreter's behavior,
 where it gives you a continuation prompt when the input is incomplete (e.g. you
 typed the start of an "if" statement or you didn't close your parentheses or
 triple string quotes), but it gives you a syntax error message immediately when
 the input is invalid.

 In Python you can use the :mod:`codeop` module, which approximates the parser's
 behavior sufficiently.  IDLE uses this, for example.

 The easiest way to do it in C is to call :cfunc:`PyRun_InteractiveLoop` (perhaps
 in a separate thread) and let the Python interpreter handle the input for
 you. You can also set the :cfunc:`PyOS_ReadlineFunctionPointer` to point at your
 custom input function. See ``Modules/readline.c`` and ``Parser/myreadline.c``
 for more hints.

 However sometimes you have to run the embedded Python interpreter in the same
 thread as your rest application and you can't allow the
 :cfunc:`PyRun_InteractiveLoop` to stop while waiting for user input.  The one
 solution then is to call :cfunc:`PyParser_ParseString` and test for ``e.error``
 equal to ``E_EOF``, which means the input is incomplete).  Here's a sample code
 fragment, untested, inspired by code from Alex Farber::

    #include <Python.h>
    #include <node.h>
    #include <errcode.h>
    #include <grammar.h>
    #include <parsetok.h>
    #include <compile.h>

    int testcomplete(char *code)
      /* code should end in \n */
      /* return -1 for error, 0 for incomplete, 1 for complete */
    {
      node *n;
      perrdetail e;

      n = PyParser_ParseString(code, &_PyParser_Grammar,
                               Py_file_input, &e);
      if (n == NULL) {
        if (e.error == E_EOF)
          return 0;
        return -1;
      }

      PyNode_Free(n);
      return 1;
    }

 Another solution is trying to compile the received string with
 :cfunc:`Py_CompileString`. If it compiles without errors, try to execute the
 returned code object by calling :cfunc:`PyEval_EvalCode`. Otherwise save the
 input for later. If the compilation fails, find out if it's an error or just
 more input is required - by extracting the message string from the exception
 tuple and comparing it to the string "unexpected EOF while parsing".  Here is a
 complete example using the GNU readline library (you may want to ignore
 **SIGINT** while calling readline())::

    #include <stdio.h>
    #include <readline.h>

    #include <Python.h>
    #include <object.h>
    #include <compile.h>
    #include <eval.h>

    int main (int argc, char* argv[])
    {
      int i, j, done = 0;                          /* lengths of line, code */
      char ps1[] = ">>> ";
      char ps2[] = "... ";
      char *prompt = ps1;
      char *msg, *line, *code = NULL;
      PyObject *src, *glb, *loc;
      PyObject *exc, *val, *trb, *obj, *dum;

      Py_Initialize ();
      loc = PyDict_New ();
      glb = PyDict_New ();
      PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());

      while (!done)
      {
        line = readline (prompt);

        if (NULL == line)                          /* CTRL-D pressed */
        {
          done = 1;
        }
        else
        {
          i = strlen (line);

          if (i > 0)
            add_history (line);                    /* save non-empty lines */

          if (NULL == code)                        /* nothing in code yet */
            j = 0;
          else
            j = strlen (code);

          code = realloc (code, i + j + 2);
          if (NULL == code)                        /* out of memory */
            exit (1);

          if (0 == j)                              /* code was empty, so */
            code[0] = '\0';                        /* keep strncat happy */

          strncat (code, line, i);                 /* append line to code */
          code[i + j] = '\n';                      /* append '\n' to code */
          code[i + j + 1] = '\0';

          src = Py_CompileString (code, "<stdin>", Py_single_input);

          if (NULL != src)                         /* compiled just fine - */
          {
            if (ps1  == prompt ||                  /* ">>> " or */
                '\n' == code[i + j - 1])           /* "... " and double '\n' */
            {                                               /* so execute it */
              dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);
              Py_XDECREF (dum);
              Py_XDECREF (src);
              free (code);
              code = NULL;
              if (PyErr_Occurred ())
                PyErr_Print ();
              prompt = ps1;
            }
          }                                        /* syntax error or E_EOF? */
          else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
          {
            PyErr_Fetch (&exc, &val, &trb);        /* clears exception! */

            if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
                !strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
            {
              Py_XDECREF (exc);
              Py_XDECREF (val);
              Py_XDECREF (trb);
              prompt = ps2;
            }
            else                                   /* some other syntax error */
            {
              PyErr_Restore (exc, val, trb);
              PyErr_Print ();
              free (code);
              code = NULL;
              prompt = ps1;
            }
          }
          else                                     /* some non-syntax error */
          {
            PyErr_Print ();
            free (code);
            code = NULL;
            prompt = ps1;
          }

          free (line);
        }
      }

      Py_XDECREF(glb);
      Py_XDECREF(loc);
      Py_Finalize();
      exit(0);
    }


 How do I find undefined g++ symbols __builtin_new or __pure_virtual?
 --------------------------------------------------------------------

 To dynamically load g++ extension modules, you must recompile Python, relink it
 using g++ (change LINKCC in the Python Modules Makefile), and link your
 extension module using g++ (e.g., ``g++ -shared -o mymodule.so mymodule.o``).


 Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?
 ----------------------------------------------------------------------------------------------------------------

 In Python 2.2, you can inherit from builtin classes such as :class:`int`,
 :class:`list`, :class:`dict`, etc.

 The Boost Python Library (BPL, http://www.boost.org/libs/python/doc/index.html)
 provides a way of doing this from C++ (i.e. you can inherit from an extension
 class written in C++ using the BPL).


 When importing module X, why do I get "undefined symbol: PyUnicodeUCS2*"?
 -------------------------------------------------------------------------

 You are using a version of Python that uses a 4-byte representation for Unicode
 characters, but some C extension module you are importing was compiled using a
 Python that uses a 2-byte representation for Unicode characters (the default).

 If instead the name of the undefined symbol starts with ``PyUnicodeUCS4``, the
 problem is the reverse: Python was built using 2-byte Unicode characters, and
 the extension module was compiled using a Python with 4-byte Unicode characters.

 This can easily occur when using pre-built extension packages.  RedHat Linux
 7.x, in particular, provided a "python2" binary that is compiled with 4-byte
 Unicode.  This only causes the link failure if the extension uses any of the
 ``PyUnicode_*()`` functions.  It is also a problem if an extension uses any of
 the Unicode-related format specifiers for :cfunc:`Py_BuildValue` (or similar) or
 parameter specifications for :cfunc:`PyArg_ParseTuple`.

 You can check the size of the Unicode character a Python interpreter is using by
 checking the value of sys.maxunicode:

    >>> import sys
    >>> if sys.maxunicode > 65535:
    ...     print('UCS4 build')
    ... else:
    ...     print('UCS2 build')

 The only way to solve this problem is to use extension modules compiled with a
 Python binary built using the same size for Unicode characters.
	=======================
	Extending/Embedding FAQ
	=======================

	.. contents::

	.. highlight:: c


	.. XXX need review for Python 3.


	Can I create my own functions in C?
	-----------------------------------

	Yes, you can create built-in modules containing functions, variables, exceptions
	and even new types in C. This is explained in the document
	:ref:`extending-index`.

	Most intermediate or advanced Python books will also cover this topic.


	Can I create my own functions in C++?
	-------------------------------------

	Yes, using the C compatibility features found in C++. Place ``extern "C" {
	... }`` around the Python include files and put ``extern "C"`` before each
	function that is going to be called by the Python interpreter. Global or static
	C++ objects with constructors are probably not a good idea.


	.. _c-wrapper-software:

	Writing C is hard; are there any alternatives?
	----------------------------------------------

	There are a number of alternatives to writing your own C extensions, depending
	on what you're trying to do.

	.. XXX make sure these all work; mention Cython

	If you need more speed, `Psyco <http://psyco.sourceforge.net/>`_ generates x86
	assembly code from Python bytecode. You can use Psyco to compile the most
	time-critical functions in your code, and gain a significant improvement with
	very little effort, as long as you're running on a machine with an
	x86-compatible processor.

	`Pyrex <http://www.cosc.canterbury.ac.nz/~greg/python/Pyrex/>`_ is a compiler
	that accepts a slightly modified form of Python and generates the corresponding
	C code. Pyrex makes it possible to write an extension without having to learn
	Python's C API.

	If you need to interface to some C or C++ library for which no Python extension
	currently exists, you can try wrapping the library's data types and functions
	with a tool such as `SWIG <http://www.swig.org>`_. `SIP
	<http://www.riverbankcomputing.co.uk/software/sip/>`__, `CXX
	<http://cxx.sourceforge.net/>`_ `Boost
	<http://www.boost.org/libs/python/doc/index.html>`_, or `Weave
	<http://www.scipy.org/Weave>`_ are also alternatives for wrapping C++ libraries.


	How can I execute arbitrary Python statements from C?
	-----------------------------------------------------

	The highest-level function to do this is :cfunc:`PyRun_SimpleString` which takes
	a single string argument to be executed in the context of the module
	``__main__`` and returns 0 for success and -1 when an exception occurred
	(including ``SyntaxError``). If you want more control, use
	:cfunc:`PyRun_String`; see the source for :cfunc:`PyRun_SimpleString` in
	``Python/pythonrun.c``.


	How can I evaluate an arbitrary Python expression from C?
	---------------------------------------------------------

	Call the function :cfunc:`PyRun_String` from the previous question with the
	start symbol :cdata:`Py_eval_input`; it parses an expression, evaluates it and
	returns its value.


	How do I extract C values from a Python object?
	-----------------------------------------------

	That depends on the object's type. If it's a tuple, :cfunc:`PyTuple_Size`
	returns its length and :cfunc:`PyTuple_GetItem` returns the item at a specified
	index. Lists have similar functions, :cfunc:`PyListSize` and
	:cfunc:`PyList_GetItem`.

	For strings, :cfunc:`PyString_Size` returns its length and
	:cfunc:`PyString_AsString` a pointer to its value. Note that Python strings may
	contain null bytes so C's :cfunc:`strlen` should not be used.

	To test the type of an object, first make sure it isn't NULL, and then use
	:cfunc:`PyString_Check`, :cfunc:`PyTuple_Check`, :cfunc:`PyList_Check`, etc.

	There is also a high-level API to Python objects which is provided by the
	so-called 'abstract' interface -- read ``Include/abstract.h`` for further
	details. It allows interfacing with any kind of Python sequence using calls
	like :cfunc:`PySequence_Length`, :cfunc:`PySequence_GetItem`, etc.) as well as
	many other useful protocols.


	How do I use Py_BuildValue() to create a tuple of arbitrary length?
	-------------------------------------------------------------------

	You can't. Use ``t = PyTuple_New(n)`` instead, and fill it with objects using
	``PyTuple_SetItem(t, i, o)`` -- note that this "eats" a reference count of
	``o``, so you have to :cfunc:`Py_INCREF` it. Lists have similar functions
	``PyList_New(n)`` and ``PyList_SetItem(l, i, o)``. Note that you must set all
	the tuple items to some value before you pass the tuple to Python code --
	``PyTuple_New(n)`` initializes them to NULL, which isn't a valid Python value.


	How do I call an object's method from C?
	----------------------------------------

	The :cfunc:`PyObject_CallMethod` function can be used to call an arbitrary
	method of an object. The parameters are the object, the name of the method to
	call, a format string like that used with :cfunc:`Py_BuildValue`, and the
	argument values::

	PyObject *
	PyObject_CallMethod(PyObject object, char method_name,
	char *arg_format, ...);

	This works for any object that has methods -- whether built-in or user-defined.
	You are responsible for eventually :cfunc:`Py_DECREF`\ 'ing the return value.

	To call, e.g., a file object's "seek" method with arguments 10, 0 (assuming the
	file object pointer is "f")::

	res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);
	if (res == NULL) {
	... an exception occurred ...
	}
	else {
	Py_DECREF(res);
	}

	Note that since :cfunc:`PyObject_CallObject` always wants a tuple for the
	argument list, to call a function without arguments, pass "()" for the format,
	and to call a function with one argument, surround the argument in parentheses,
	e.g. "(i)".


	How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?
	----------------------------------------------------------------------------------------

	In Python code, define an object that supports the ``write()`` method. Assign
	this object to :data:`sys.stdout` and :data:`sys.stderr`. Call print_error, or
	just allow the standard traceback mechanism to work. Then, the output will go
	wherever your ``write()`` method sends it.

	The easiest way to do this is to use the StringIO class in the standard library.

	Sample code and use for catching stdout:

	>>> class StdoutCatcher:
	... def __init__(self):
	... self.data = ''
	... def write(self, stuff):
	... self.data = self.data + stuff
	...
	>>> import sys
	>>> sys.stdout = StdoutCatcher()
	>>> print('foo')
	>>> print('hello world!')
	>>> sys.stderr.write(sys.stdout.data)
	foo
	hello world!


	How do I access a module written in Python from C?
	--------------------------------------------------

	You can get a pointer to the module object as follows::

	module = PyImport_ImportModule("<modulename>");

	If the module hasn't been imported yet (i.e. it is not yet present in
	:data:`sys.modules`), this initializes the module; otherwise it simply returns
	the value of ``sys.modules["<modulename>"]``. Note that it doesn't enter the
	module into any namespace -- it only ensures it has been initialized and is
	stored in :data:`sys.modules`.

	You can then access the module's attributes (i.e. any name defined in the
	module) as follows::

	attr = PyObject_GetAttrString(module, "<attrname>");

	Calling :cfunc:`PyObject_SetAttrString` to assign to variables in the module
	also works.


	How do I interface to C++ objects from Python?
	----------------------------------------------

	Depending on your requirements, there are many approaches. To do this manually,
	begin by reading :ref:`the "Extending and Embedding" document
	<extending-index>`. Realize that for the Python run-time system, there isn't a
	whole lot of difference between C and C++ -- so the strategy of building a new
	Python type around a C structure (pointer) type will also work for C++ objects.

	For C++ libraries, you can look at `SIP
	<http://www.riverbankcomputing.co.uk/sip/>`_, `CXX
	<http://cxx.sourceforge.net/>`_, `Boost
	<http://www.boost.org/libs/python/doc/index.html>`_, `Weave
	<http://www.scipy.org/Weave>`_ or `SWIG <http://www.swig.org>`_


	I added a module using the Setup file and the make fails; why?
	--------------------------------------------------------------

	Setup must end in a newline, if there is no newline there, the build process
	fails. (Fixing this requires some ugly shell script hackery, and this bug is so
	minor that it doesn't seem worth the effort.)


	How do I debug an extension?
	----------------------------

	When using GDB with dynamically loaded extensions, you can't set a breakpoint in
	your extension until your extension is loaded.

	In your ``.gdbinit`` file (or interactively), add the command::

	br _PyImport_LoadDynamicModule

	Then, when you run GDB::

	$ gdb /local/bin/python
	gdb) run myscript.py
	gdb) continue # repeat until your extension is loaded
	gdb) finish # so that your extension is loaded
	gdb) br myfunction.c:50
	gdb) continue

	I want to compile a Python module on my Linux system, but some files are missing. Why?
	--------------------------------------------------------------------------------------

	Most packaged versions of Python don't include the
	:file:`/usr/lib/python2.{x}/config/` directory, which contains various files
	required for compiling Python extensions.

	For Red Hat, install the python-devel RPM to get the necessary files.

	For Debian, run ``apt-get install python-dev``.


	What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean?
	-------------------------------------------------------------------------------------

	This means that you have created an extension module named "yourmodule", but
	your module init function does not initialize with that name.

	Every module init function will have a line similar to::

	module = Py_InitModule("yourmodule", yourmodule_functions);

	If the string passed to this function is not the same name as your extension
	module, the :exc:`SystemError` exception will be raised.


	How do I tell "incomplete input" from "invalid input"?
	------------------------------------------------------

	Sometimes you want to emulate the Python interactive interpreter's behavior,
	where it gives you a continuation prompt when the input is incomplete (e.g. you
	typed the start of an "if" statement or you didn't close your parentheses or
	triple string quotes), but it gives you a syntax error message immediately when
	the input is invalid.

	In Python you can use the :mod:`codeop` module, which approximates the parser's
	behavior sufficiently. IDLE uses this, for example.

	The easiest way to do it in C is to call :cfunc:`PyRun_InteractiveLoop` (perhaps
	in a separate thread) and let the Python interpreter handle the input for
	you. You can also set the :cfunc:`PyOS_ReadlineFunctionPointer` to point at your
	custom input function. See ``Modules/readline.c`` and ``Parser/myreadline.c``
	for more hints.

	However sometimes you have to run the embedded Python interpreter in the same
	thread as your rest application and you can't allow the
	:cfunc:`PyRun_InteractiveLoop` to stop while waiting for user input. The one
	solution then is to call :cfunc:`PyParser_ParseString` and test for ``e.error``
	equal to ``E_EOF``, which means the input is incomplete). Here's a sample code
	fragment, untested, inspired by code from Alex Farber::

	#include <Python.h>
	#include <node.h>
	#include <errcode.h>
	#include <grammar.h>
	#include <parsetok.h>
	#include <compile.h>

	int testcomplete(char *code)
	/* code should end in \n */
	/* return -1 for error, 0 for incomplete, 1 for complete */
	{
	node *n;
	perrdetail e;

	n = PyParser_ParseString(code, &_PyParser_Grammar,
	Py_file_input, &e);
	if (n == NULL) {
	if (e.error == E_EOF)
	return 0;
	return -1;
	}

	PyNode_Free(n);
	return 1;
	}

	Another solution is trying to compile the received string with
	:cfunc:`Py_CompileString`. If it compiles without errors, try to execute the
	returned code object by calling :cfunc:`PyEval_EvalCode`. Otherwise save the
	input for later. If the compilation fails, find out if it's an error or just
	more input is required - by extracting the message string from the exception
	tuple and comparing it to the string "unexpected EOF while parsing". Here is a
	complete example using the GNU readline library (you may want to ignore
	SIGINT while calling readline())::

	#include <stdio.h>
	#include <readline.h>

	#include <Python.h>
	#include <object.h>
	#include <compile.h>
	#include <eval.h>

	int main (int argc, char* argv[])
	{
	int i, j, done = 0; /* lengths of line, code */
	char ps1[] = ">>> ";
	char ps2[] = "... ";
	char *prompt = ps1;
	char msg, line, *code = NULL;
	PyObject src, glb, *loc;
	PyObject exc, val, trb, obj, *dum;

	Py_Initialize ();
	loc = PyDict_New ();
	glb = PyDict_New ();
	PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());

	while (!done)
	{
	line = readline (prompt);

	if (NULL == line) /* CTRL-D pressed */
	{
	done = 1;
	}
	else
	{
	i = strlen (line);

	if (i > 0)
	add_history (line); /* save non-empty lines */

	if (NULL == code) /* nothing in code yet */
	j = 0;
	else
	j = strlen (code);

	code = realloc (code, i + j + 2);
	if (NULL == code) /* out of memory */
	exit (1);

	if (0 == j) /* code was empty, so */
	code[0] = '\0'; /* keep strncat happy */

	strncat (code, line, i); /* append line to code */
	code[i + j] = '\n'; /* append '\n' to code */
	code[i + j + 1] = '\0';

	src = Py_CompileString (code, "<stdin>", Py_single_input);

	if (NULL != src) /* compiled just fine - */
	{
	if (ps1 == prompt \|\| /* ">>> " or */
	'\n' == code[i + j - 1]) /* "... " and double '\n' */
	{ /* so execute it */
	dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);
	Py_XDECREF (dum);
	Py_XDECREF (src);
	free (code);
	code = NULL;
	if (PyErr_Occurred ())
	PyErr_Print ();
	prompt = ps1;
	}
	} /* syntax error or E_EOF? */
	else if (PyErr_ExceptionMatches (PyExc_SyntaxError))
	{
	PyErr_Fetch (&exc, &val, &trb); /* clears exception! */

	if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&
	!strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */
	{
	Py_XDECREF (exc);
	Py_XDECREF (val);
	Py_XDECREF (trb);
	prompt = ps2;
	}
	else /* some other syntax error */
	{
	PyErr_Restore (exc, val, trb);
	PyErr_Print ();
	free (code);
	code = NULL;
	prompt = ps1;
	}
	}
	else /* some non-syntax error */
	{
	PyErr_Print ();
	free (code);
	code = NULL;
	prompt = ps1;
	}

	free (line);
	}
	}

	Py_XDECREF(glb);
	Py_XDECREF(loc);
	Py_Finalize();
	exit(0);
	}


	How do I find undefined g++ symbols __builtin_new or __pure_virtual?
	--------------------------------------------------------------------

	To dynamically load g++ extension modules, you must recompile Python, relink it
	using g++ (change LINKCC in the Python Modules Makefile), and link your
	extension module using g++ (e.g., ``g++ -shared -o mymodule.so mymodule.o``).


	Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)?
	----------------------------------------------------------------------------------------------------------------

	In Python 2.2, you can inherit from builtin classes such as :class:`int`,
	:class:`list`, :class:`dict`, etc.

	The Boost Python Library (BPL, http://www.boost.org/libs/python/doc/index.html)
	provides a way of doing this from C++ (i.e. you can inherit from an extension
	class written in C++ using the BPL).


	When importing module X, why do I get "undefined symbol: PyUnicodeUCS2*"?
	-------------------------------------------------------------------------

	You are using a version of Python that uses a 4-byte representation for Unicode
	characters, but some C extension module you are importing was compiled using a
	Python that uses a 2-byte representation for Unicode characters (the default).

	If instead the name of the undefined symbol starts with ``PyUnicodeUCS4``, the
	problem is the reverse: Python was built using 2-byte Unicode characters, and
	the extension module was compiled using a Python with 4-byte Unicode characters.

	This can easily occur when using pre-built extension packages. RedHat Linux
	7.x, in particular, provided a "python2" binary that is compiled with 4-byte
	Unicode. This only causes the link failure if the extension uses any of the
	``PyUnicode_*()`` functions. It is also a problem if an extension uses any of
	the Unicode-related format specifiers for :cfunc:`Py_BuildValue` (or similar) or
	parameter specifications for :cfunc:`PyArg_ParseTuple`.

	You can check the size of the Unicode character a Python interpreter is using by
	checking the value of sys.maxunicode:

	>>> import sys
	>>> if sys.maxunicode > 65535:
	... print('UCS4 build')
	... else:
	... print('UCS2 build')

	The only way to solve this problem is to use extension modules compiled with a
	Python binary built using the same size for Unicode characters.