Edgewall Software

Markup Streams

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

1   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)

2   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()

3   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.

4   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')
>>> print substream.select('text()')
a link

See also: genshi.core, Documentation

Last modified 3 years ago Last modified on Dec 14, 2015 6:22:32 AM