====================
BeautifulSoup Parser
====================

BeautifulSoup_ is a Python package that parses broken HTML, just like
lxml supports it based on the parser of libxml2.  BeautifulSoup uses a
different parsing approach.  It is not a real HTML parser but uses
regular expressions to dive through tag soup.  It is therefore more
forgiving in some cases and less good in others.  It is not uncommon
that lxml/libxml2 parses and fixes broken HTML better, but
BeautifulSoup has superiour `support for encoding detection`_.  It
very much depends on the input which parser works better.

.. _BeautifulSoup: http://www.crummy.com/software/BeautifulSoup/
.. _`support for encoding detection`: http://www.crummy.com/software/BeautifulSoup/documentation.html#Beautiful%20Soup%20Gives%20You%20Unicode%2C%20Dammit
.. _ElementSoup: http://effbot.org/zone/element-soup.htm

To prevent users from having to choose their parser library in
advance, lxml can interface to the parsing capabilities of
BeautifulSoup through the ``lxml.html.soupparser`` module.  It
provides three main functions: ``fromstring()`` and ``parse()`` to
parse a string or file using BeautifulSoup into an ``lxml.html``
document, and ``convert_tree()`` to convert an existing BeautifulSoup
tree into a list of top-level Elements.

.. contents::
..
   1  Parsing with the soupparser
   2  Entity handling
   3  Using soupparser as a fallback
   4  Using only the encoding detection


Parsing with the soupparser
===========================

The functions ``fromstring()`` and ``parse()`` behave as known from
ElementTree.  The first returns a root Element, the latter returns an
ElementTree.

There is also a legacy module called ``lxml.html.ElementSoup``, which
mimics the interface provided by ElementTree's own ElementSoup_
module.  Note that the ``soupparser`` module was added in lxml 2.0.3.
Previous versions of lxml 2.0.x only have the ``ElementSoup`` module.

Here is a document full of tag soup, similar to, but not quite like, HTML:

.. sourcecode:: pycon

    >>> tag_soup = '<meta><head><title>Hello</head<body onload=crash()>Hi all<p>'

all you need to do is pass it to the ``fromstring()`` function:

.. sourcecode:: pycon

    >>> from lxml.html.soupparser import fromstring
    >>> root = fromstring(tag_soup)

To see what we have here, you can serialise it:

.. sourcecode:: pycon

    >>> from lxml.etree import tostring
    >>> print tostring(root, pretty_print=True),
    <html>
      <meta/>
      <head>
        <title>Hello</title>
      </head>
      <body onload="crash()">Hi all<p/></body>
    </html>

Not quite what you'd expect from an HTML page, but, well, it was broken
already, right?  BeautifulSoup did its best, and so now it's a tree.

To control which Element implementation is used, you can pass a
``makeelement`` factory function to ``parse()`` and ``fromstring()``.
By default, this is based on the HTML parser defined in ``lxml.html``.

For a quick comparison, libxml2 2.6.32 parses the same tag soup as
follows.  The main difference is that libxml2 tries harder to adhere
to the structure of an HTML document and moves misplaced tags where
they (likely) belong.  Note, however, that the result can vary between
parser versions.

.. sourcecode:: html

    <html>
      <head>
        <meta/>
        <title>Hello</title>
      </head>
      <body>
        <p>Hi all</p>
        <p/>
      </body>
    </html>


Entity handling
===============

By default, the BeautifulSoup parser also replaces the entities it
finds by their character equivalent.

.. sourcecode:: pycon

    >>> tag_soup = '<body>&copy;&euro;&#45;&#245;&#445;<p>'
    >>> body = fromstring(tag_soup).find('.//body')
    >>> body.text
    u'\xa9\u20ac-\xf5\u01bd'

If you want them back on the way out, you can just serialise with the
default encoding, which is 'US-ASCII'.

.. sourcecode:: pycon

    >>> tostring(body)
    '<body>&#169;&#8364;-&#245;&#445;<p/></body>'

    >>> tostring(body, method="html")
    '<body>&#169;&#8364;-&#245;&#445;<p></p></body>'

Any other encoding will output the respective byte sequences.

.. sourcecode:: pycon

    >>> tostring(body, encoding="utf-8")
    '<body>\xc2\xa9\xe2\x82\xac-\xc3\xb5\xc6\xbd<p/></body>'

    >>> tostring(body, method="html", encoding="utf-8")
    '<body>\xc2\xa9\xe2\x82\xac-\xc3\xb5\xc6\xbd<p></p></body>'

    >>> tostring(body, encoding=unicode)
    u'<body>\xa9\u20ac-\xf5\u01bd<p/></body>'

    >>> tostring(body, method="html", encoding=unicode)
    u'<body>\xa9\u20ac-\xf5\u01bd<p></p></body>'


Using soupparser as a fallback
==============================

The downside of using this parser is that it is `much slower`_ than
the HTML parser of lxml.  So if performance matters, you might want to
consider using ``soupparser`` only as a fallback for certain cases.

.. _`much slower`: http://blog.ianbicking.org/2008/03/30/python-html-parser-performance/

One common problem of lxml's parser is that it might not get the
encoding right in cases where the document contains a ``<meta>`` tag
at the wrong place.  In this case, you can exploit the fact that lxml
serialises much faster than most other HTML libraries for Python.
Just serialise the document to unicode and if that gives you an
exception, re-parse it with BeautifulSoup to see if that works
better.

.. sourcecode:: pycon

    >>> tag_soup = '''\
    ... <meta http-equiv="Content-Type"
    ...       content="text/html;charset=utf-8" />
    ... <html>
    ...   <head>
    ...     <title>Hello W\xc3\xb6rld!</title>
    ...   </head>
    ...   <body>Hi all</body>
    ... </html>'''

    >>> import lxml.html
    >>> import lxml.html.soupparser

    >>> root = lxml.html.fromstring(tag_soup)
    >>> try:
    ...     ignore = tostring(root, encoding=unicode)
    ... except UnicodeDecodeError:
    ...     root = lxml.html.soupparser.fromstring(tag_soup)


Using only the encoding detection
=================================

If you prefer a 'real' (and fast) HTML parser instead of the regular
expression based one in BeautifulSoup, you can still benefit from
BeautifulSoup's `support for encoding detection`_ in the
``UnicodeDammit`` class.

.. sourcecode:: pycon

    >>> from BeautifulSoup import UnicodeDammit

    >>> def decode_html(html_string):
    ...     converted = UnicodeDammit(html_string, isHTML=True)
    ...     if not converted.unicode:
    ...         raise UnicodeDecodeError(
    ...             "Failed to detect encoding, tried [%s]",
    ...             ', '.join(converted.triedEncodings))
    ...     # print converted.originalEncoding
    ...     return converted.unicode

    >>> root = lxml.html.fromstring(decode_html(tag_soup))
