doc/lxmlhtml.txt

=========
lxml.html
=========

:Author:
  Ian Bicking

Since version 2.0, lxml comes with a dedicated Python package for
dealing with HTML: ``lxml.html``.  It is based on lxml's HTML parser,
but provides a special Element API for HTML elements, as well as a
number of utilities for common HTML processing tasks.

.. contents::
.. 
   1  Parsing HTML
     1.1  Parsing HTML fragments
     1.2  Really broken pages
   2  HTML Element Methods
   3  Running HTML doctests
   4  Creating HTML with the E-factory
     4.1  Viewing your HTML
   5  Working with links
     5.1  Functions
   6  Forms
     6.1  Form Filling Example
     6.2  Form Submission
   7  Cleaning up HTML
     7.1  autolink
     7.2  wordwrap
   8  HTML Diff
   9  Examples
     9.1  Microformat Example

The main API is based on the `lxml.etree`_ API, and thus, on the ElementTree_
API.

.. _`lxml.etree`: tutorial.html
.. _ElementTree:  http://effbot.org/zone/element-index.htm


Parsing HTML
============

Parsing HTML fragments
----------------------

There are several functions available to parse HTML:

``parse(filename_url_or_file)``:
    Parses the named file or url, or if the object has a ``.read()``
    method, parses from that.

    If you give a URL, or if the object has a ``.geturl()`` method (as
    file-like objects from ``urllib.urlopen()`` have), then that URL
    is used as the base URL.  You can also provide an explicit
    ``base_url`` keyword argument.

``document_fromstring(string)``:
    Parses a document from the given string.  This always creates a
    correct HTML document, which means the parent node is ``<html>``,
    and there is a body and possibly a head.

``fragment_fromstring(string, create_parent=False)``:
    Returns an HTML fragment from a string.  The fragment must contain
    just a single element, unless ``create_parent`` is given;
    e.g., ``fragment_fromstring(string, create_parent='div')`` will
    wrap the element in a ``<div>``.

``fragments_fromstring(string)``:
    Returns a list of the elements found in the fragment.

``fromstring(string)``:
    Returns ``document_fromstring`` or ``fragment_fromstring``, based
    on whether the string looks like a full document, or just a
    fragment.

Really broken pages
-------------------

The normal HTML parser is capable of handling broken HTML, but for
pages that are far enough from HTML to call them 'tag soup', it may
still fail to parse the page in a useful way.  A way to deal with this
is ElementSoup_, which deploys the well-known BeautifulSoup_ parser to
build an lxml HTML tree.

.. _BeautifulSoup: http://www.crummy.com/software/BeautifulSoup/
.. _ElementSoup: elementsoup.html

However, note that the most common problem with web pages is the lack
of (or the existence of incorrect) encoding declarations.  It is
therefore often sufficient to only use the encoding detection of
BeautifulSoup, called UnicodeDammit, and to leave the rest to lxml's
own HTML parser, which is several times faster.


HTML Element Methods
====================

HTML elements have all the methods that come with ElementTree, but
also include some extra methods:

``.drop_tree()``:
    Drops the element and all its children.  Unlike
    ``el.getparent().remove(el)`` this does *not* remove the tail
    text; with ``drop_tree`` the tail text is merged with the previous
    element.

``.drop_tag()``:
    Drops the tag, but keeps its children and text.

``.find_class(class_name)``:
    Returns a list of all the elements with the given CSS class name.
    Note that class names are space separated in HTML, so
    ``doc.find_class_name('highlight')`` will find an element like
    ``<div class="sidebar highlight">``.  Class names *are* case
    sensitive.

``.find_rel_links(rel)``:
    Returns a list of all the ``<a rel="{rel}">`` elements.  E.g.,
    ``doc.find_rel_links('tag')`` returns all the links `marked as
    tags <http://microformats.org/wiki/rel-tag>`_.

``.get_element_by_id(id, default=None)``:
    Return the element with the given ``id``, or the ``default`` if
    none is found.  If there are multiple elements with the same id
    (which there shouldn't be, but there often is), this returns only
    the first.

``.text_content()``:
    Returns the text content of the element, including the text
    content of its children, with no markup.

``.cssselect(expr)``:
    Select elements from this element and its children, using a CSS
    selector expression.  (Note that ``.xpath(expr)`` is also
    available as on all lxml elements.)

``.label``:
    Returns the corresponding ``<label>`` element for this element, if
    any exists (None if there is none).  Label elements have a
    ``label.for_element`` attribute that points back to the element.

``.base_url``:
    The base URL for this element, if one was saved from the parsing.
    This attribute is not settable.  Is None when no base URL was
    saved.

``.classes``:
    Returns a set-like object that allows accessing and modifying the
    names in the 'class' attribute of the element.  (New in lxml 3.5).

``.set(key, value=None)``:
    Sets an HTML attribute.  If no value is given, or if the value is
    ``None``, it creates a boolean attribute like ``<form novalidate></form>``
    or ``<div custom-attribute></div>``.  In XML, attributes must
    have at least the empty string as their value like ``<form
    novalidate=""></form>``, but HTML boolean attributes can also be
    just present or absent from an element without having a value.

Running HTML doctests
=====================

One of the interesting modules in the ``lxml.html`` package deals with
doctests.  It can be hard to compare two HTML pages for equality, as
whitespace differences aren't meaningful and the structural formatting
can differ.  This is even more a problem in doctests, where output is
tested for equality and small differences in whitespace or the order
of attributes can let a test fail.  And given the verbosity of
tag-based languages, it may take more than a quick look to find the
actual differences in the doctest output.

Luckily, lxml provides the ``lxml.doctestcompare`` module that
supports relaxed comparison of XML and HTML pages and provides a
readable diff in the output when a test fails.  The HTML comparison is
most easily used by importing the ``usedoctest`` module in a doctest:

.. sourcecode:: pycon

    >>> import lxml.html.usedoctest

Now, if you have an HTML document and want to compare it to an expected result
document in a doctest, you can do the following:

.. sourcecode:: pycon

    >>> import lxml.html
    >>> html = lxml.html.fromstring('''\
    ...    <html><body onload="" color="white">
    ...      <p>Hi  !</p>
    ...    </body></html>
    ... ''')

    >>> print lxml.html.tostring(html)
    <html><body onload="" color="white"><p>Hi !</p></body></html>

    >>> print lxml.html.tostring(html)
    <html> <body color="white" onload=""> <p>Hi    !</p> </body> </html>

    >>> print lxml.html.tostring(html)
    <html>
      <body color="white" onload="">
        <p>Hi !</p>
      </body>
    </html>

In documentation, you would likely prefer the pretty printed HTML output, as
it is the most readable.  However, the three documents are equivalent from the
point of view of an HTML tool, so the doctest will silently accept any of the
above.  This allows you to concentrate on readability in your doctests, even
if the real output is a straight ugly HTML one-liner.

Note that there is also an ``lxml.usedoctest`` module which you can
import for XML comparisons.  The HTML parser notably ignores
namespaces and some other XMLisms.


Creating HTML with the E-factory
================================

.. _`E-factory`: http://online.effbot.org/2006_11_01_archive.htm#et-builder

lxml.html comes with a predefined HTML vocabulary for the `E-factory`_,
originally written by Fredrik Lundh.  This allows you to quickly generate HTML
pages and fragments:

.. sourcecode:: pycon

    >>> from lxml.html import builder as E
    >>> from lxml.html import usedoctest
    >>> html = E.HTML(
    ...   E.HEAD(
    ...     E.LINK(rel="stylesheet", href="great.css", type="text/css"),
    ...     E.TITLE("Best Page Ever")
    ...   ),
    ...   E.BODY(
    ...     E.H1(E.CLASS("heading"), "Top News"),
    ...     E.P("World News only on this page", style="font-size: 200%"),
    ...     "Ah, and here's some more text, by the way.",
    ...     lxml.html.fromstring("<p>... and this is a parsed fragment ...</p>")
    ...   )
    ... )

    >>> print lxml.html.tostring(html)
    <html>
      <head>
        <link href="great.css" rel="stylesheet" type="text/css">
        <title>Best Page Ever</title>
      </head>
      <body>
        <h1 class="heading">Top News</h1>
        <p style="font-size: 200%">World News only on this page</p>
        Ah, and here's some more text, by the way.
        <p>... and this is a parsed fragment ...</p>
      </body>
    </html>

Note that you should use ``lxml.html.tostring`` and **not**
``lxml.tostring``.  ``lxml.tostring(doc)`` will return the XML
representation of the document, which is not valid HTML.  In
particular, things like ``<script src="..."></script>`` will be
serialized as ``<script src="..." />``, which completely confuses
browsers.

Viewing your HTML
-----------------

A handy method for viewing your HTML:
``lxml.html.open_in_browser(lxml_doc)`` will write the document to
disk and open it in a browser (with the `webbrowser module
<http://python.org/doc/current/lib/module-webbrowser.html>`_).

Working with links
==================

There are several methods on elements that allow you to see and modify
the links in a document.

``.iterlinks()``:
    This yields ``(element, attribute, link, pos)`` for every link in
    the document.  ``attribute`` may be None if the link is in the
    text (as will be the case with a ``<style>`` tag with
    ``@import``).  

    This finds any link in an ``action``, ``archive``, ``background``,
    ``cite``, ``classid``, ``codebase``, ``data``, ``href``,
    ``longdesc``, ``profile``, ``src``, ``usemap``, ``dynsrc``, or
    ``lowsrc`` attribute.  It also searches ``style`` attributes for
    ``url(link)``, and ``<style>`` tags for ``@import`` and ``url()``.

    This function does *not* pay attention to ``<base href>``.

``.resolve_base_href()``:
    This function will modify the document in-place to take account of
    ``<base href>`` if the document contains that tag.  In the process
    it will also remove that tag from the document.

``.make_links_absolute(base_href, resolve_base_href=True)``:
    This makes all links in the document absolute, assuming that
    ``base_href`` is the URL of the document.  So if you pass
    ``base_href="http://localhost/foo/bar.html"`` and there is a link
    to ``baz.html`` that will be rewritten as
    ``http://localhost/foo/baz.html``.

    If ``resolve_base_href`` is true, then any ``<base href>`` tag
    will be taken into account (just calling
    ``self.resolve_base_href()``).

``.rewrite_links(link_repl_func, resolve_base_href=True, base_href=None)``:
    This rewrites all the links in the document using your given link
    replacement function.  If you give a ``base_href`` value, all
    links will be passed in after they are joined with this URL.

    For each link ``link_repl_func(link)`` is called.  That function
    then returns the new link, or None to remove the attribute or tag
    that contains the link.  Note that all links will be passed in,
    including links like ``"#anchor"`` (which is purely internal), and
    things like ``"mailto:bob@example.com"`` (or ``javascript:...``).

    If you want access to the context of the link, you should use
    ``.iterlinks()`` instead.

Functions
---------

In addition to these methods, there are corresponding functions:

* ``iterlinks(html)``
* ``make_links_absolute(html, base_href, ...)``
* ``rewrite_links(html, link_repl_func, ...)``
* ``resolve_base_href(html)``

These functions will parse ``html`` if it is a string, then return the new
HTML as a string.  If you pass in a document, the document will be copied
(except for ``iterlinks()``), the method performed, and the new document
returned.

Forms
=====

Any ``<form>`` elements in a document are available through
the list ``doc.forms`` (e.g., ``doc.forms[0]``).  Form, input, select,
and textarea elements each have special methods.

Input elements (including ``<select>`` and ``<textarea>``) have these
attributes:

``.name``:
    The name of the element.

``.value``:
    The value of an input, the content of a textarea, the selected
    option(s) of a select.  This attribute can be set.  

    In the case of a select that takes multiple options (``<select
    multiple>``) this will be a set of the selected options; you can
    add or remove items to select and unselect the options.

Select attributes:

``.value_options``:
    For select elements, this is all the *possible* values (the values
    of all the options).

``.multiple``:
    For select elements, true if this is a ``<select multiple>``
    element.

Input attributes:

``.type``:
    The type attribute in ``<input>`` elements.

``.checkable``:
    True if this can be checked (i.e., true for type=radio and
    type=checkbox).

``.checked``:
    If this element is checkable, the checked state.  Raises
    AttributeError on non-checkable inputs.

The form itself has these attributes:

``.inputs``:
    A dictionary-like object that can be used to access input elements
    by name.  When there are multiple input elements with the same
    name, this returns list-like structures that can also be used to
    access the options and their values as a group.

``.fields``:
    A dictionary-like object used to access *values* by their name.
    ``form.inputs`` returns elements, this only returns values.
    Setting values in this dictionary will effect the form inputs.
    Basically ``form.fields[x]`` is equivalent to
    ``form.inputs[x].value`` and ``form.fields[x] = y`` is equivalent
    to ``form.inputs[x].value = y``.  (Note that sometimes
    ``form.inputs[x]`` returns a compound object, but these objects
    also have ``.value`` attributes.)

    If you set this attribute, it is equivalent to
    ``form.fields.clear(); form.fields.update(new_value)``

``.form_values()``:
    Returns a list of ``[(name, value), ...]``, suitable to be passed
    to ``urllib.urlencode()`` for form submission.

``.action``:
    The ``action`` attribute.  This is resolved to an absolute URL if
    possible.

``.method``:
    The ``method`` attribute, which defaults to ``GET``.

Form Filling Example
--------------------

Note that you can change any of these attributes (values, method,
action, etc) and then serialize the form to see the updated values.
You can, for instance, do:

.. sourcecode:: pycon

    >>> from lxml.html import fromstring, tostring
    >>> form_page = fromstring('''<html><body><form>
    ...   Your name: <input type="text" name="name"> <br>
    ...   Your phone: <input type="text" name="phone"> <br>
    ...   Your favorite pets: <br>
    ...   Dogs: <input type="checkbox" name="interest" value="dogs"> <br>
    ...   Cats: <input type="checkbox" name="interest" value="cats"> <br>
    ...   Llamas: <input type="checkbox" name="interest" value="llamas"> <br>
    ...   <input type="submit"></form></body></html>''')
    >>> form = form_page.forms[0]
    >>> form.fields = dict(
    ...     name='John Smith',
    ...     phone='555-555-3949',
    ...     interest=set(['cats', 'llamas']))
    >>> print(tostring(form))
    <html>
      <body>
        <form>
        Your name:
          <input name="name" type="text" value="John Smith">
          <br>Your phone:
          <input name="phone" type="text" value="555-555-3949">
          <br>Your favorite pets:
          <br>Dogs:
          <input name="interest" type="checkbox" value="dogs">
          <br>Cats:
          <input checked name="interest" type="checkbox" value="cats">
          <br>Llamas:
          <input checked name="interest" type="checkbox" value="llamas">
          <br>
          <input type="submit">
        </form>
      </body>
    </html>


Form Submission
---------------

You can submit a form with ``lxml.html.submit_form(form_element)``.
This will return a file-like object (the result of
``urllib.urlopen()``).

If you have extra input values you want to pass you can use the
keyword argument ``extra_values``, like ``extra_values={'submit':
'Yes!'}``.  This is the only way to get submit values into the form,
as there is no state of "submitted" for these elements.

You can pass in an alternate opener with the ``open_http`` keyword
argument, which is a function with the signature ``open_http(method,
url, values)``.

Example:

.. sourcecode:: pycon

    >>> from lxml.html import parse, submit_form
    >>> page = parse('http://tinyurl.com').getroot()
    >>> page.forms[0].fields['url'] = 'http://lxml.de/'
    >>> result = parse(submit_form(page.forms[0])).getroot()

    >>> [a.attrib['href'] for a in result.xpath("//a[@target='_blank']")]
    ['http://tinyurl.com/2xae8s', 'http://preview.tinyurl.com/2xae8s']


HTML Diff
=========

The module ``lxml.html.diff`` offers some ways to visualize
differences in HTML documents.  These differences are *content*
oriented.  That is, changes in markup are largely ignored; only
changes in the content itself are highlighted.

There are two ways to view differences: ``htmldiff`` and
``html_annotate``.  One shows differences with ``<ins>`` and
``<del>``, while the other annotates a set of changes similar to ``svn
blame``.  Both these functions operate on text, and work best with
content fragments (only what goes in ``<body>``), not complete
documents.

Example of ``htmldiff``:

.. sourcecode:: pycon

    >>> from lxml.html.diff import htmldiff, html_annotate
    >>> doc1 = '''<p>Here is some text.</p>'''
    >>> doc2 = '''<p>Here is <b>a lot</b> of <i>text</i>.</p>'''
    >>> doc3 = '''<p>Here is <b>a little</b> <i>text</i>.</p>'''
    >>> print htmldiff(doc1, doc2)
    <p>Here is <ins><b>a lot</b> of <i>text</i>.</ins> <del>some text.</del> </p>
    >>> print html_annotate([(doc1, 'author1'), (doc2, 'author2'),
    ...                      (doc3, 'author3')])
    <p><span title="author1">Here is</span>
       <b><span title="author2">a</span>
       <span title="author3">little</span></b>
       <i><span title="author2">text</span></i>
       <span title="author2">.</span></p>

As you can see, it is imperfect as such things tend to be.  On larger
tracts of text with larger edits it will generally do better.

The ``html_annotate`` function can also take an optional second
argument, ``markup``.  This is a function like ``markup(text,
version)`` that returns the given text marked up with the given
version.  The default version, the output of which you see in the
example, looks like:

.. sourcecode:: python

    def default_markup(text, version):
        return '<span title="%s">%s</span>' % (
            cgi.escape(unicode(version), 1), text)

Examples
========

Microformat Example
-------------------

This example parses the `hCard <http://microformats.org/wiki/hcard>`_
microformat.

First we get the page:

.. sourcecode:: pycon

    >>> import urllib
    >>> from lxml.html import fromstring
    >>> url = 'http://microformats.org/'
    >>> content = urllib.urlopen(url).read()
    >>> doc = fromstring(content)
    >>> doc.make_links_absolute(url)

Then we create some objects to put the information in:

.. sourcecode:: pycon

    >>> class Card(object):
    ...     def __init__(self, **kw):
    ...         for name, value in kw:
    ...             setattr(self, name, value)
    >>> class Phone(object):
    ...     def __init__(self, phone, types=()):
    ...         self.phone, self.types = phone, types

And some generally handy functions for microformats:

.. sourcecode:: pycon

    >>> def get_text(el, class_name):
    ...     els = el.find_class(class_name)
    ...     if els:
    ...         return els[0].text_content()
    ...     else:
    ...         return ''
    >>> def get_value(el):
    ...     return get_text(el, 'value') or el.text_content()
    >>> def get_all_texts(el, class_name):
    ...     return [e.text_content() for e in els.find_class(class_name)]
    >>> def parse_addresses(el):
    ...     # Ideally this would parse street, etc.
    ...     return el.find_class('adr')

Then the parsing:

.. sourcecode:: pycon

    >>> for el in doc.find_class('hcard'):
    ...     card = Card()
    ...     card.el = el
    ...     card.fn = get_text(el, 'fn')
    ...     card.tels = []
    ...     for tel_el in card.find_class('tel'):
    ...         card.tels.append(Phone(get_value(tel_el),
    ...                                get_all_texts(tel_el, 'type')))
    ...     card.addresses = parse_addresses(el)