FAQ sobre Extensão/Incorporação
*******************************


Posso criar minhas próprias funções em C?
=========================================

Sim, você pode construir módulos embutidos contendo funções,
variáveis, exceções e até mesmo novos tipos em C. Isso é explicado no
documento Estendendo e Incorporando o Intérprete de Python.

A maioria dos livros intermediários ou avançados em Python também
abordará esse tópico.


Posso criar minhas próprias funções em C++?
===========================================

Sim, usando recursos de compatibilidade encontrados em C++. Coloque ``
extern "C" {...} `` em torno dos arquivos de inclusão do Python e
coloque `` extern "C" `` antes de cada função que será chamada pelo
interpretador do Python. Objetos globais ou estáticos em C ++ com
construtores provavelmente não são uma boa ideia.


A escrita em C é difícil, Há algumas alternativas?
==================================================

Há um número de alternativas para escrever suas próprias extensões em
C, dependendo daquilo que você está tentando fazer.

Cython and its relative Pyrex are compilers that accept a slightly
modified form of Python and generate the corresponding C code.  Cython
and Pyrex make 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.  SIP, CXX Boost, or
Weave are also alternatives for wrapping C++ libraries.


Como posso executar instruções arbitrárias de Python a partir de C?
===================================================================

The highest-level function to do this is "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 "PyRun_String()"; see the source for
"PyRun_SimpleString()" in "Python/pythonrun.c".


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

Call the function "PyRun_String()" from the previous question with the
start symbol "Py_eval_input"; it parses an expression, evaluates it
and returns its value.


Como extraio valores em C a partir de um objeto Python?
=======================================================

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

For bytes, "PyBytes_Size()" returns its length and
"PyBytes_AsStringAndSize()" provides a pointer to its value and its
length.  Note that Python bytes objects may contain null bytes so C's
"strlen()" should not be used.

To test the type of an object, first make sure it isn't "NULL", and
then use "PyBytes_Check()", "PyTuple_Check()", "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 "PySequence_Length()",
"PySequence_GetItem()", etc. as well as many other useful protocols
such as numbers ("PyNumber_Index()" et al.) and mappings in the
PyMapping APIs.


Como posso utilizar Py_BuildValue() para criar uma tupla de comprimento arbitrário?
===================================================================================

You can't.  Use "PyTuple_Pack()" instead.


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

The "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 "Py_BuildValue()",
and the argument values:

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

This works for any object that has methods -- whether built-in or
user-defined. You are responsible for eventually "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 "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 "sys.stdout" and "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 "io.StringIO" class:

   >>> import io, sys
   >>> sys.stdout = io.StringIO()
   >>> print('foo')
   >>> print('hello world!')
   >>> sys.stderr.write(sys.stdout.getvalue())
   foo
   hello world!

Um objeto personalizado para fazer a mesma coisa seria esse:

   >>> import io, sys
   >>> class StdoutCatcher(io.TextIOBase):
   ...     def __init__(self):
   ...         self.data = []
   ...     def write(self, stuff):
   ...         self.data.append(stuff)
   ...
   >>> import sys
   >>> sys.stdout = StdoutCatcher()
   >>> print('foo')
   >>> print('hello world!')
   >>> sys.stderr.write(''.join(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
"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 "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 "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 the "Extending and Embedding" document.
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, see A escrita em C é difícil, Há algumas
alternativas?.


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
"/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".


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 "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 "PyRun_InteractiveLoop()"
(perhaps in a separate thread) and let the Python interpreter handle
the input for you. You can also set the
"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
"PyRun_InteractiveLoop()" to stop while waiting for user input.  The
one solution then is to call "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:

   #define PY_SSIZE_T_CLEAN
   #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
"Py_CompileString()". If it compiles without errors, try to execute
the returned code object by calling "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>

   #define PY_SSIZE_T_CLEAN
   #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 (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)?
================================================================================================================

Yes, you can inherit from built-in classes such as "int", "list",
"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).
