~jtaylor/ubuntu/oneiric/genshi/dh_python2

.. -*- mode: rst; encoding: utf-8 -*-

==============
Markup Streams
==============

A stream is the common representation of markup as a *stream of events*.


.. contents:: Contents
   :depth: 2
.. sectnum::


Basics
======

A stream can be attained in a number of ways. It can be:

* the result of parsing XML or HTML text, or
* programmatically generated, or
* the result of selecting a subset of another stream filtered by an XPath
  expression.

For example, the functions ``XML()`` and ``HTML()`` can be used to convert
literal XML or HTML text to a markup stream::

  >>> from genshi import XML
  >>> stream = XML('<p class="intro">Some text and '
  ...              '<a href="http://example.org/">a link</a>.'
  ...              '<br/></p>')
  >>> stream
  <genshi.core.Stream object at 0x6bef0>

The stream is the result of parsing the text into events. Each event is a tuple
of the form ``(kind, data, pos)``, where:

* ``kind`` defines what kind of event it is (such as the start of an element,
  text, a comment, etc).
* ``data`` is the actual data associated with the event. How this looks depends
  on the event kind.
* ``pos`` is a ``(filename, lineno, column)`` tuple that describes where the
  event “comes from”.

::

  >>> for kind, data, pos in stream:
  ...     print kind, `data`, pos
  ... 
  START (u'p', [(u'class', u'intro')]) ('<string>', 1, 0)
  TEXT u'Some text and ' ('<string>', 1, 31)
  START (u'a', [(u'href', u'http://example.org/')]) ('<string>', 1, 31)
  TEXT u'a link' ('<string>', 1, 67)
  END u'a' ('<string>', 1, 67)
  TEXT u'.' ('<string>', 1, 72)
  START (u'br', []) ('<string>', 1, 72)
  END u'br' ('<string>', 1, 77)
  END u'p' ('<string>', 1, 77)


Filtering
=========

One important feature of markup streams is that you can apply *filters* to the
stream, either filters that come with Genshi, or your own custom filters.

A filter is simply a callable that accepts the stream as parameter, and returns
the filtered stream::

  def noop(stream):
      """A filter that doesn't actually do anything with the stream."""
      for kind, data, pos in stream:
          yield kind, data, pos

Filters can be applied in a number of ways. The simplest is to just call the
filter directly::

  stream = noop(stream)

The ``Stream`` class also provides a ``filter()`` method, which takes an
arbitrary number of filter callables and applies them all::

  stream = stream.filter(noop)

Finally, filters can also be applied using the *bitwise or* operator (``|``),
which allows a syntax similar to pipes on Unix shells::

  stream = stream | noop

One example of a filter included with Genshi is the ``HTMLSanitizer`` in
``genshi.filters``. It processes a stream of HTML markup, and strips out any
potentially dangerous constructs, such as Javascript event handlers.
``HTMLSanitizer`` is not a function, but rather a class that implements
``__call__``, which means instances of the class are callable.

Both the ``filter()`` method and the pipe operator allow easy chaining of
filters::

  from genshi.filters import HTMLSanitizer
  stream = stream.filter(noop, HTMLSanitizer())

That is equivalent to::

  stream = stream | noop | HTMLSanitizer()


Serialization
=============

The ``Stream`` class provides two methods for serializing this list of events:
``serialize()`` and ``render()``. The former is a generator that yields chunks
of ``Markup`` objects (which are basically unicode strings that are considered
safe for output on the web). The latter returns a single string, by default
UTF-8 encoded.

Here's the output from ``serialize()``::

  >>> for output in stream.serialize():
  ...     print `output`
  ... 
  <Markup u'<p class="intro">'>
  <Markup u'Some text and '>
  <Markup u'<a href="http://example.org/">'>
  <Markup u'a link'>
  <Markup u'</a>'>
  <Markup u'.'>
  <Markup u'<br/>'>
  <Markup u'</p>'>

And here's the output from ``render()``::

  >>> print stream.render()
  <p class="intro">Some text and <a href="http://example.org/">a link</a>.<br/></p>

Both methods can be passed a ``method`` parameter that determines how exactly
the events are serialzed to text. This parameter can be either “xml” (the
default), “xhtml”, “html”, “text”, or a custom serializer class::

  >>> print stream.render('html')
  <p class="intro">Some text and <a href="http://example.org/">a link</a>.<br></p>

Note how the `<br>` element isn't closed, which is the right thing to do for
HTML.

In addition, the ``render()`` method takes an ``encoding`` parameter, which
defaults to “UTF-8”. If set to ``None``, the result will be a unicode string.

The different serializer classes in ``genshi.output`` can also be used
directly::

  >>> from genshi.filters import HTMLSanitizer
  >>> from genshi.output import TextSerializer
  >>> print TextSerializer()(HTMLSanitizer()(stream))
  Some text and a link.

The pipe operator allows a nicer syntax::

  >>> print stream | HTMLSanitizer() | TextSerializer()
  Some text and a link.

Using XPath
===========

XPath can be used to extract a specific subset of the stream via the
``select()`` method::

  >>> substream = stream.select('a')
  >>> substream
  <genshi.core.Stream object at 0x7118b0>
  >>> print substream
  <a href="http://example.org/">a link</a>

Often, streams cannot be reused: in the above example, the sub-stream is based
on a generator. Once it has been serialized, it will have been fully consumed,
and cannot be rendered again. To work around this, you can wrap such a stream
in a ``list``::

  >>> from genshi import Stream
  >>> substream = Stream(list(stream.select('a')))
  >>> substream
  <genshi.core.Stream object at 0x7118b0>
  >>> print substream
  <a href="http://example.org/">a link</a>
  >>> print substream.select('@href')
  http://example.org/
  >>> print substream.select('text()')
  a link