C Extension Module using Python
Last Updated :
27 Mar, 2019
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Writing a simple C extension module directly using Python’s extension API and no other tools. It is straightforward to make a handcrafted extension module for a simple C code. But first, we have to make sure that the C code has a proper header file.
Code #1 :
C
The header would correspond to a library that has been separately compiled. The code below illustrates the basics of writing extension functions, following this assumption.
Code #2:
CPP 1==
Code #3 : Module method table and structure
CPP 1==
Code #4: Creating a
Python3
Code #5: Now simply use python3 buildlib.py
Python3
Output :
Python3
#include <math.h>
extern int gcd(int, int);
extern int in_mandel(double x0, double y0, int n);
extern int divide(int a, int b, int *remainder);
extern double avg(double *a, int n);
typedef struct Point
{
double x, y;
} Point;
extern double distance(Point *p1, Point *p2);
# include "Python.h"
# include "sample.h"
/* int gcd(int, int) */
static PyObject * py_gcd(PyObject * self, PyObject * args)
{
int x, y, result;
if (! PyArg_ParseTuple(args, "ii", &x, &y))
{
return NULL;
}
result = gcd(x, y);
return Py_BuildValue("i", result);
}
/* int divide(int, int, int *) */
static PyObject * py_divide(PyObject * self, PyObject * args)
{
int a, b, quotient, remainder;
if (! PyArg_ParseTuple(args, "ii", &a, &b))
{
return NULL;
}
quotient = divide(a, b, &remainder);
return Py_BuildValue("(ii)", quotient, remainder);
}
/* Module method table */
static PyMethodDef SampleMethods[] =
{
{"gcd", py_gcd, METH_VARARGS, "Greatest common divisor"},
{"divide", py_divide, METH_VARARGS, "Integer division"},
{ NULL, NULL, 0, NULL}
};
/* Module structure */
static struct PyModuleDef samplemodule =
{
PyModuleDef_HEAD_INIT,
"sample", /* name of module */
"A sample module", /* Doc string (may be NULL) */
-1, /* Size of per-interpreter state or -1 */
SampleMethods /* Method table */
};
/* Module initialization function */
PyMODINIT_FUNC
PyInit_sample(void)
{
return PyModule_Create(&samplemodule);
}
setup.py python file for building the extension module.
# setup.py
from distutils.core import setup, Extension
setup(name='sample',
ext_modules=[
Extension('sample',
['pysample.c'],
include_dirs = ['/some/dir'],
define_macros = [('FOO','1')],
undef_macros = ['BAR'],
library_dirs = ['/usr/local/lib'],
libraries = ['sample']
)
]
)
build_ext --inplace, to build the resulting library.
bash% python3 setup.py build_ext --inplace running build_ext building 'sample' extension gcc -fno-strict-aliasing -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/local/include/python3.3m -c pysample.c -o build/temp.macosx-10.6-x86_64-3.3/pysample.o gcc -bundle -undefined dynamic_lookup build/temp.macosx-10.6-x86_64-3.3/pysample.o \ -L/usr/local/lib -lsample -o sample.so bash %The above code will create a shared library called sample.so. Code #6 :
import sample
print ("gcd = ", sample.gcd(35, 42))
print ("\ndistance : ", sample.divide(42, 8))
gcd = 7 distance = (5, 2)“Extending and Embedding the Python Interpreter” is a Python's documentation that can be consulted before attempting any kind of handwritten extension. In extension modules, functions can be written as shown in code snippet below. Code #4 :
static PyObject *py_func(PyObject *self, PyObject *args)
{
...
}
- PyObject - C data type that represents any Python object. At a very high level, an extension function is a C function that receives a tuple of Python objects (in
PyObject *args) and returns a new Python object as a result. The self argument to the function is unused for simple extension functions, but comes into play should you want to define new classes or object types in C. - The
PyArg_ParseTuple()function is used to convert values from Python to a C representation. As input, it takes a format string that indicates the required values, such as “i” for integer and “d” for double, as well as the addresses of C variables in which to place the converted results. Py_BuildValue()function is used to create Python objects from C data types. It also accepts a format code to indicate the desired type. In the extension functions, it is used to return results back to Python. One feature ofPy_BuildValue()is that it can build more complicated kinds of objects, such as tuples and dictionaries.
