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How to read the source of lxml
Stefan Behnel
.. meta::
:description: How to read and work on the source code of lxml
:keywords: lxml, XML, Cython, source code, develop, comprehend, understand
This document describes how to read the source code of lxml_ and how
to start working on it. You might also be interested in the companion
document that describes `how to build lxml from sources`_.
.. _lxml:
.. _`how to build lxml from sources`: build.html
.. _`ReStructured Text`:
.. _epydoc:
.. _docutils:
.. _`C-level API`: capi.html
.. contents::
1 What is Cython?
2 Where to start?
2.1 Concepts
2.2 The documentation
3 lxml.etree
4 lxml.objectify
5 lxml.html
What is Cython?
.. _Cython:
.. _Pyrex:
Cython_ is the language that lxml is written in. It is a very
Python-like language that was specifically designed for writing Python
extension modules.
The reason why Cython (or actually its predecessor Pyrex_ at the time)
was chosen as an implementation language for lxml, is that it makes it
very easy to interface with both the Python world and external C code.
Cython generates all the necessary glue code for the Python API,
including Python types, calling conventions and reference counting.
On the other side of the table, calling into C code is not more than
declaring the signature of the function and maybe some variables as
being C types, pointers or structs, and then calling it. The rest of
the code is just plain Python code.
The Cython language is so close to Python that the Cython compiler can
actually compile many, many Python programs to C without major
modifications. But the real speed gains of a C compilation come from
type annotations that were added to the language and that allow Cython
to generate very efficient C code.
Even if you are not familiar with Cython, you should keep in mind that
a slow implementation of a feature is better than none. So, if you
want to contribute and have an idea what code you want to write, feel
free to start with a pure Python implementation. Chances are, if you
get the change officially accepted and integrated, others will take
the time to optimise it so that it runs fast in Cython.
Where to start?
First of all, read `how to build lxml from sources`_ to learn how to
retrieve the source code from the Subversion repository and how to
build it. The source code lives in the subdirectory ``src`` of the
The main extension modules in lxml are ``lxml.etree`` and
``lxml.objectify``. All main modules have the file extension
``.pyx``, which shows the descendence from Pyrex. As usual in Python,
the main files start with a short description and a couple of imports.
Cython distinguishes between the run-time ``import`` statement (as
known from Python) and the compile-time ``cimport`` statement, which
imports C declarations, either from external libraries or from other
Cython modules.
lxml's tree API is based on proxy objects. That means, every Element
object (or rather ``_Element`` object) is a proxy for a libxml2 node
structure. The class declaration is (mainly)::
cdef class _Element:
cdef _Document _doc
cdef xmlNode* _c_node
It is a naming convention that C variables and C level class members
that are passed into libxml2 start with a prefixed ``c_`` (commonly
libxml2 struct pointers), and that C level class members are prefixed
with an underscore. So you will often see names like ``c_doc`` for an
``xmlDoc*`` variable (or ``c_node`` for an ``xmlNode*``), or the above
``_c_node`` for a class member that points to an ``xmlNode`` struct
(or ``_c_doc`` for an ``xmlDoc*``).
It is important to know that every proxy in lxml has a factory
function that properly sets up C level members. Proxy objects must
*never* be instantiated outside of that factory. For example, to
instantiate an _Element object or its subclasses, you must always call
its factory function::
cdef xmlNode* c_node
cdef _Document doc
cdef _Element element
element = _elementFactory(doc, c_node)
A good place to see how this factory is used are the Element methods
``getparent()``, ``getnext()`` and ``getprevious()``.
The documentation
An important part of lxml is the documentation that lives in the
``doc`` directory. It describes a large part of the API and comprises
a lot of example code in the form of doctests.
The documentation is written in the `ReStructured Text`_ format, a
very powerful text markup language that looks almost like plain text.
It is part of the docutils_ package.
The project web site of lxml_ is completely generated from these text
documents. Even the side menu is just collected from the table of
contents that the ReST processor writes into each HTML page.
Obviously, we use lxml for this.
The easiest way to generate the HTML pages is by calling::
make html
This will call the script ``doc/`` to run the ReST processor
on the files. After generating an HTML page the script parses it back
in to build the side menu, and injects the complete menu into each
page at the very end.
Running the ``make`` command will also generate the API documentation
if you have epydoc_ installed. The epydoc package will import and
introspect the extension modules and also introspect and parse the
Python modules of lxml. The aggregated information will then be
written out into an HTML documentation site.
The main module, ``lxml.etree``, is in the file `lxml.etree.pyx
<>`_. It
implements the main functions and types of the ElementTree API, as
well as all the factory functions for proxies. It is the best place
to start if you want to find out how a specific feature is
At the very end of the file, it contains a series of ``include``
statements that merge the rest of the implementation into the
generated C code. Yes, you read right: no importing, no source file
namespacing, just plain good old include and a huge C code result of
more than 100,000 lines that we throw right into the C compiler.
The main include files are:
Private C helper functions. Except for the factory functions,
most of the little functions that are used all over the place are
defined here. This includes things like reading out the text
content of a libxml2 tree node, checking input from the API level,
creating a new Element node or handling attribute values. If you
want to work on the lxml code, you should keep these functions in
the back of your head, as they will definitely make your life
Element class lookup mechanisms. The main API and engines for
those who want to define custom Element classes and inject them
into lxml.
Support for custom document loaders. Base class and registry for
custom document resolvers.
Infrastructure for extension functions in XPath/XSLT, including
XPath value conversion and function registration.
Incremental XML parsing. An iterator class that builds iterparse
events while parsing.
Namespace implementation and registry. The registry and engine
for Element classes that use the ElementNamespaceClassLookup
Parsers for XML and HTML. This is the main parser engine. It's
the reason why you can parse a document from various sources in
two lines of Python code. It's definitely not the right place to
start reading lxml's soure code.
An ElementTree compatible parser target implementation based on
the SAX2 interface of libxml2.
Very low-level functions for memory allocation/deallocation
and Element proxy handling. Ignoring this for the beginning
will safe your head from exploding.
The set of C functions that are exported to other extension
modules at the C level. For example, ``lxml.objectify`` makes use
of these. See the `C-level API` documentation.
A separate read-only implementation of the Element API. This is
used in places where non-intrusive access to a tree is required,
such as the ``PythonElementClassLookup`` or XSLT extension
SAX-like parser interfaces as known from ElementTree's TreeBuilder.
XML output functions. Basically everything that creates byte
sequences from XML trees.
XInclude support.
Error log handling. All error messages that libxml2 generates
internally walk through the code in this file to end up in lxml's
Python level error logs.
At the end of the file, you will find a long list of named error
codes. It is generated from the libxml2 HTML documentation (using
lxml, of course). See the script ````
for this.
XMLID and IDDict, a dictionary-like way to find Elements by their
XML-ID attribute.
XPath evaluators.
XSL transformations, including the ``XSLT`` class, document lookup
handling and access control.
The different schema languages (DTD, RelaxNG, XML Schema and
Schematron) are implemented in the following include files:
* dtd.pxi
* relaxng.pxi
* schematron.pxi
* xmlschema.pxi
Python modules
The ``lxml`` package also contains a number of pure Python modules:
The E-factory and the ElementBuilder class. These provide a
simple interface to XML tree generation.
A CSS selector implementation based on XPath. The main class is
called ``CSSSelector``.
A relaxed comparison scheme for XML/HTML markup in doctest.
XInclude-like document inclusion, compatible with ElementTree.
XPath-like path language, compatible with ElementTree.
SAX2 compatible interfaces to copy lxml trees from/to SAX compatible
Wrapper module for ```` that simplifies its usage
from inside a doctest.
A Cython implemented extension module that uses the public C-API of
lxml.etree. It provides a Python object-like interface to XML trees.
The implementation resides in the file `lxml.objectify.pyx
A specialised toolkit for HTML handling, based on lxml.etree. This is
implemented in pure Python.
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