py.xml: simple pythonic xml/html file generation


There are a plethora of frameworks and libraries to generate xml and html trees. However, many of them are large, have a steep learning curve and are often hard to debug. Not to speak of the fact that they are frameworks to begin with.

a pythonic object model , please

The py lib offers a pythonic way to generate xml/html, based on ideas from xist which uses python class objects to build xml trees. However, xist’s implementation is somewhat heavy because it has additional goals like transformations and supporting many namespaces. But its basic idea is very easy.

generating arbitrary xml structures

With py.xml.Namespace you have the basis to generate custom xml-fragments on the fly:

class ns(py.xml.Namespace):
    "my custom xml namespace"
doc = ns.books("May Day"),
        ns.title("python for java programmers"),),"why"),
        ns.title("Java for Python programmers"),),
print doc.unicode(indent=2).encode('utf8')

will give you this representation:

<books publisher="N.N">
    <author>May Day</author>
    <title>python for java programmers</title></book>
    <title>Java for Python programmers</title></book></books>

In a sentence: positional arguments are child-tags and keyword-arguments are attributes.

On a side note, you’ll see that the unicode-serializer supports a nice indentation style which keeps your generated html readable, basically through emulating python’s white space significance by putting closing-tags rightmost and almost invisible at first glance :-)

basic example for generating html

Consider this example:

from py.xml import html  # html namespace

paras = "First Para", "Second para"

doc = html.html(
        html.meta(name="Content-Type", value="text/html; charset=latin1")),
        [html.p(p) for p in paras]))

print unicode(doc).encode('latin1')

Again, tags are objects which contain tags and have attributes. More exactly, Tags inherit from the list type and thus can be manipulated as list objects. They additionally support a default way to represent themselves as a serialized unicode object.

If you happen to look at the py.xml implementation you’ll note that the tag/namespace implementation consumes some 50 lines with another 50 lines for the unicode serialization code.

CSS-styling your html Tags

One aspect where many of the huge python xml/html generation frameworks utterly fail is a clean and convenient integration of CSS styling. Often, developers are left alone with keeping CSS style definitions in sync with some style files represented as strings (often in a separate .css file). Not only is this hard to debug but the missing abstractions make it hard to modify the styling of your tags or to choose custom style representations (inline, html.head or external). Add the Browers usual tolerance of messyness and errors in Style references and welcome to hell, known as the domain of developing web applications :-)

By contrast, consider this CSS styling example:

class my(html):
    "my initial custom style"
    class body(html.body):
        style = html.Style(font_size = "120%")

    class h2(html.h2):
        style = html.Style(background = "grey")

    class p(html.p):
        style = html.Style(font_weight="bold")

doc = my.html(
        my.h2("hello world"),
        my.p("bold as bold can")

print doc.unicode(indent=2)

This will give you a small’n mean self contained represenation by default:

  <body style="font-size: 120%">
    <h2 style="background: grey">hello world</h2>
    <p style="font-weight: bold">bold as bold can</p></body></html>

Most importantly, note that the inline-styling is just an implementation detail of the unicode serialization code. You can easily modify the serialization to put your styling into the html.head or in a separate file and autogenerate CSS-class names or ids.

Hey, you could even write tests that you are using correct styles suitable for specific browser requirements. Did i mention that the ability to easily write tests for your generated html and its serialization could help to develop _stable_ user interfaces?

More to come …

For now, i don’t think we should strive to offer much more than the above. However, it is probably not hard to offer partial serialization to allow generating maybe hundreds of complex html documents per second. Basically we would allow putting callables both as Tag content and as values of attributes. A slightly more advanced Serialization would then produce a list of unicode objects intermingled with callables. At HTTP-Request time the callables would get called to complete the probably request-specific serialization of your Tags. Hum, it’s probably harder to explain this than to actually code it :-)