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

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

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


.. contents:: Contents
   :depth: 1
.. 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 ...>

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 (see  `event kinds`_)
* ``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 (QName(u'p'), Attrs([(QName(u'class'), u'intro')])) (None, 1, 0)
  TEXT u'Some text and ' (None, 1, 17)
  START (QName(u'a'), Attrs([(QName(u'href'), u'http://example.org/')])) (None, 1, 31)
  TEXT u'a link' (None, 1, 61)
  END QName(u'a') (None, 1, 67)
  TEXT u'.' (None, 1, 71)
  START (QName(u'br'), Attrs()) (None, 1, 72)
  END QName(u'br') (None, 1, 77)
  END QName(u'p') (None, 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 ''.join(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 ...>
  >>> 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 ...>
  >>> print substream
  <a href="http://example.org/">a link</a>
  >>> print substream.select('@href')
  http://example.org/
  >>> print substream.select('text()')
  a link

See `Using XPath in Genshi`_ for more information about the XPath support in
Genshi.

.. _`Using XPath in Genshi`: xpath.html


.. _`event kinds`:

Event Kinds
===========

Every event in a stream is of one of several *kinds*, which also determines
what the ``data`` item of the event tuple looks like. The different kinds of
events are documented below.

.. note:: The ``data`` item is generally immutable. It the data is to be
   modified when processing a stream, it must be replaced by a new tuple.
   Effectively, this means the entire event tuple is immutable.

START
-----
The opening tag of an element.

For this kind of event, the ``data`` item is a tuple of the form
``(tagname, attrs)``, where ``tagname`` is a ``QName`` instance describing the
qualified name of the tag, and ``attrs`` is an ``Attrs`` instance containing
the attribute names and values associated with the tag (excluding namespace
declarations)::

  START, (QName(u'p'), Attrs([(u'class', u'intro')])), pos

END
---
The closing tag of an element.

The ``data`` item of end events consists of just a ``QName`` instance
describing the qualified name of the tag::

  END, QName(u'p'), pos

TEXT
----
Character data outside of elements and other nodes.

For text events, the ``data`` item should be a unicode object::

  TEXT, u'Hello, world!', pos

START_NS
--------
The start of a namespace mapping, binding a namespace prefix to a URI.

The ``data`` item of this kind of event is a tuple of the form
``(prefix, uri)``, where ``prefix`` is the namespace prefix and ``uri`` is the
full URI to which the prefix is bound. Both should be unicode objects. If the
namespace is not bound to any prefix, the ``prefix`` item is an empty string::

  START_NS, (u'svg', u'http://www.w3.org/2000/svg'), pos

END_NS
------
The end of a namespace mapping.

The ``data`` item of such events consists of only the namespace prefix (a
unicode object)::

  END_NS, u'svg', pos

DOCTYPE
-------
A document type declaration.

For this type of event, the ``data`` item is a tuple of the form
``(name, pubid, sysid)``, where ``name`` is the name of the root element,
``pubid`` is the public identifier of the DTD (or ``None``), and ``sysid`` is
the system identifier of the DTD (or ``None``)::

  DOCTYPE, (u'html', u'-//W3C//DTD XHTML 1.0 Transitional//EN', \
            u'http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd'), pos

COMMENT
-------
A comment.

For such events, the ``data`` item is a unicode object containing all character
data between the comment delimiters::

  COMMENT, u'Commented out', pos

PI
--
A processing instruction.

The ``data`` item is a tuple of the form ``(target, data)`` for processing
instructions, where ``target`` is the target of the PI (used to identify the
application by which the instruction should be processed), and ``data`` is text
following the target (excluding the terminating question mark)::

  PI, (u'php', u'echo "Yo" '), pos

START_CDATA
-----------
Marks the beginning of a ``CDATA`` section.

The ``data`` item for such events is always ``None``::

  START_CDATA, None, pos

END_CDATA
---------
Marks the end of a ``CDATA`` section.

The ``data`` item for such events is always ``None``::

  END_CDATA, None, pos