

In [1]:

    
from IPython.display import display, Markdown;



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def print_code(path, lang=""):
    with open(path) as f:
        return Markdown("```%s\n%s```" % (lang,f.read()));

Notes: Interfacing with C

Taken from Scipy: Interfacing with C.

Python-C-API

The Python-C-API is the backbone of the standard Python interpreter. Using this module it is possible to write Python extension modules in C and C++. Using this module often requires significant boilerplate code to convert between datatypes.

Using the Python-C-API is not recommended in practice, but it is useful to understand because other methods rely on the concepts in this section.

Advantages

Requires no additional libraries
Lots of low-level control
Entirely usable from C++

Disadvantages

May require a substantial amount of effort
Significant overhead
Must be compiled
High maintenance cost
No forward compatibility across Python versions as C-API changes
Reference count bugs are easy to create and hard to find

C-extension module

We will create an extension module based on the following C code, which simply wraps the cos method from math.h.



In [3]:

    
print_code("python-c-api/cos_module.c", "c")









    Out[3]:




/*  Example of wrapping cos function from math.h with the Python-C-API. */

#include <Python.h>
#include <math.h>

/*  wrapped cosine function */
static PyObject* cos_func(PyObject* self, PyObject* args) {
    double value;
    double answer;

    /*  parse the input, from python float to c double */
    if (!PyArg_ParseTuple(args, "d", &value))
        return NULL;
    /* if the above function returns -1, an appropriate Python exception will
     * have been set, and the function simply returns NULL
     */

    /* call cos from libm */
    answer = cos(value);

    /*  construct the output from cos, from c double to python float */
    return Py_BuildValue("f", answer);
}

/*  define functions in module */
static PyMethodDef CosMethods[] = {
     {"cos_func", cos_func, METH_VARARGS, "evaluate the cosine"},
     {NULL, NULL, 0, NULL}
};

/* module initialization */
PyMODINIT_FUNC initcos_module(void) {
     (void) Py_InitModule("cos_module", CosMethods);
}

Compiling the Extension

The following setup.py file uses distutils to compile the C extension.



In [4]:

    
print_code("python-c-api/setup.py", "python")









    Out[4]:




from distutils.core import setup, Extension;

# define the extension module
cos_module = Extension('cos_module', sources=['cos_module.c']);
# run setup
setup(ext_modules=[cos_module]);

We now call setup.py with the following options:

build_ext builds extension modules
--inplace will output the compiled extension module into the current directory



In [12]:

    
%%bash
cd python-c-api
python setup.py build_ext --inplace









    



running build_ext

Now we see that a new .so file has been created:



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%%bash
ls









    



build
python-c-api
Scipy - Interfacing Python with C.ipynb

Importing the Extension



In [17]:

    
import cos_module.cpython-35m-x86_64-linux-gnu as cos_module









    



  File "<ipython-input-17-1f8a4809caec>", line 1
    import cos_module.cpython-35m-x86_64-linux-gnu as cos_module
                             ^
SyntaxError: invalid syntax



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