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# The public C-API of lxml.etree

As of version 1.1, lxml.etree provides a public C-API. This allows external C extensions to efficiently access public functions and classes of lxml, without going through the Python API.

The API is described in the file etreepublic.pxd, which is directly c-importable by extension modules implemented in Pyrex or Cython.

Contents

- Writing external modules in Cython
- Writing external modules in C

# 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(
         etree.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.