Python Bindings Overview

Python Bindings Overview
RudáMoura : ruda.moura@gmail.com
Sébastien Tandel : sebastien.tandel@gmail.com

Python C API
PythonBrasil[5]
Caxias do Sul, RS - Brazil

3
Python C API : philosophy
Everything is a PyObject*
Public symbols with Py prefix
Private symbols with _Py prefix
Objects are stored in memory heap
It’s you job to count the references
Py_INCREF & Py_DECREF
Yeah, that’s sucks!
There are functions to convert C data types to Python C data types and back
PyArg_ParseTuple()
Py_BuildValue()

Python C API
A wrapper around two readline functions:
readline - return a malloced string
add_history - add a (const) string to the history
char *readline(const char *);
intadd_history(const char *);

Python C API
readline()
#include <Python.h>
#include <readline/readline.h>
PyObject *
fn_readline(PyObject *self, PyObject *args)
{
char *prompt;
char *line;
PyObject *o;
if (!PyArg_ParseTuple(args, "s", &prompt))
return NULL;
line = readline(prompt);
o = Py_BuildValue("s", line);
free(line);
return o;
}

Python C API
add_history()
PyObject *
fn_add_history(PyObject *self, PyObject *args)
{
char *line;
int status;
PyObject *o;
if (!PyArg_ParseTuple(args, "s", &line))
return NULL;
status = add_history(line);
o = Py_BuildValue("i", status);
return o;
}
And if I want to return None?
Py_INCREF(Py_None);
return Py_None;

Python C API
Registering functions and modules
static PyMethodDef
methods[] = {
{"readline", fn_readline, METH_VARARGS, NULL},
{"add_history", fn_add_history, METH_VARARGS, NULL},
{NULL, NULL, 0, NULL}
};
PyMODINIT_FUNC
initreadline(void)
{
(void) Py_InitModule("readline", methods);
}

Python C API
Compiling...
$ gcc -dynamiclib -I/usr/include/python2.5 -lpython -lreadline mod_readline.c -o readline.so
Or you can use distutils / setup.py

Python C API
The goods
Strong C/C++ and Python integration
The best performance possible
Full Python data types support
The bads
It’s hard work, only good for masochists
You’re supposed to be good in C
Reference counts nightmare

Python C API : summary

SWIG
Simplified Wrapper and Interface Generator

SWIG : philosophy
Everything is defined in the interface file
Module name
Literal #include or source code
Define What symbols to export
The swig compiler translates the interface file into C source code

SWIG
The interface file
// readline.i
%module readline
%{
#include <readline/readline.h>
%}
char *readline(const char *);
intadd_history(const char *);

SWIG
Compiling...
$ swig -python readline.i
$ gcc -creadline_wrap.c -I/usr/include/python2.5
$ gcc -dynamiclibreadline_wrap.o -lpython -lreadline -o _readline.so
Or you can use distutils / setup.py

SWIG
The goods
Full bindings process automatization
Strong C++ support (class, template, exception)
Not only for Python! Support for Perl, Ruby and others
It’s mature (since 1995)
The bads
The C file created is not for human consum
Callbacks and double references are not easy

SWIG : summary

CTYPES

Ctypes : philosophy
Ctypes is a dlopen-like for Python
The basic steps :
Obtain a handle of a library
Use this handle to access library’s functions
from ctypes import cdll
libc_h = cdll.LoadLibrary(“libc.so.6”)
libc_h.mkdir(“python-mkdir-test”)

Ctypes
OK, it’s a dlopen-like, and?
libc_h.mkdir(“python-mkdir-test”)
It’s a Python string!
Transparent conversion for these types :
None, integers, long, byte strings and unicode strings

Ctypes
What if I want specific C types?
c_int, c_long, c_char, c_short, c_ulong, …
i = c_int(10)
print i.value()
10

Ctypes
Functions return values?
By default, assumed to be int
from ctypes import cdll, c_char_p
lr = cdll.LoadLibrary("libreadline.dylib")
while True:
line = lr.readline(“C:>")
c_line = c_char_p(line)
if not c_line.value: break

Ctypes
Return value as input for another C function?
while True:
line = lr.readline(“C:>")
c_line = c_char_p(line)
if not c_line.value: break
lr.add_history(c_line.value) # or
lr.add_history(c_line) # or
lr.add_history(line)

Ctypes
Is there a way to simplify a bit?
lr.readline.restype = c_char_p
while True:
line = lr.readline("c:>")
if not line: break
lr.add_history(line)
You can define types for arguments too with .argtypes!

Ctypes
Possible to access structures returned by C functions
Possible to define arrays … but with a fixed length!
Possible to create callback written in Python called by C lib.
On windows, check the number of parameters passed to a function.
Part of Python 2.5

Ctypes : summary

Cython

Cython : philosophy
Cython is an extension of Python language
Three simple steps :
Write “python”
Compile the module it with C compiler
Import the package in your python code

Cython
Compiling a module => setuptools
print “hello world!”
setup.py
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext
setup(
cmdclass = {'build_ext': build_ext},
ext_modules = [Extension(”hello", [”hello.pyx"])]
)

Cython
cdef extern from "readline/readline.h":
char* readline(char*)
intadd_history(char*)
def readline_loop():
while True:
line = readline("c:>”)
if not line: break
add_history(line)

Cython
setup.py
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext
setup(
cmdclass = {'build_ext': build_ext},
ext_modules = [Extension(‘readlineloop’,
[‘readline-loop.pyx’],
libraries = [‘readline’])]
)

Cython
Last details :
Can access C structures
Possible to define callbacks
Check on number of args
Possible to wrap C++ classes!
Handle exceptions!
Can be used to optimize your code
need to learn a bit more

Cython
Some limitations :
No support to yield
No nested def
No globals(), locals()
Class / func can’t be placed inside control structures

Cython : summary

Conclusions

Thank You!
ruda.moura@gmail.com
sebastien.tandel@gmail.com

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Python Bindings Overview

by Sebastien Tandel

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