Writing external modules in Cython

This is the easiest way of extending lxml at the C level. A Cython (or Pyrex) module should start like this:

# My Cython extension

# import the public functions and classes of lxml.etree
cimport etreepublic as cetree

# import the lxml.etree module in Python
cdef object etree
from lxml import etree

# initialize the access to the C-API of lxml.etree
cetree.import_lxml__etree()

From this line on, you can access all public functions of lxml.etree from the cetree namespace like this:

# build a tag name from namespace and element name
py_tag = cetree.namespacedNameFromNsName("http://some/url", "myelement")

Public lxml classes are easily subclassed. For example, to implement and set a new default element class, you can write Cython code like the following:

from etreepublic cimport ElementBase
cdef class NewElementClass(ElementBase):
     def set_value(self, myval):
         self.set("my_attribute", myval)

etree.set_element_class_lookup(
     DefaultElementClassLookup(element=NewElementClass))

Writing external modules in C

If you really feel like it, you can also interface with lxml.etree straight from C code. All you have to do is include the header file for the public API, import the lxml.etree module and then call the import function:

/* My C extension */

/* common includes */
#include "Python.h"
#include "stdio.h"
#include "string.h"
#include "stdarg.h"
#include "libxml/xmlversion.h"
#include "libxml/encoding.h"
#include "libxml/hash.h"
#include "libxml/tree.h"
#include "libxml/xmlIO.h"
#include "libxml/xmlsave.h"
#include "libxml/globals.h"
#include "libxml/xmlstring.h"

/* lxml.etree specific includes */
#include "lxml-version.h"
#include "etree_defs.h"
#include "etree.h"

/* setup code */
import_lxml__etree()

Note that including etree.h does not automatically include the header files it requires. Note also that the above list of common includes may not be sufficient.