Getting lxml 2 into my gnarly old Tiger; going on ROA

In case anyone might google for something like:

lxml "Symbol not found" _xmlSchematronNewParserCtxt

, I'd like to reiterate the steps I just took to get lxml 2 (2.1 beta 1 to be precise) up and running on OS X 10.4 (which I haven't yet purged in favour of the shiny Leopard for reasons of fear, uncertainty and doubt. And while pleasurable, quite mixed impressions from that cat when running my wicked new iMac (and my funky old iBook, in case you're dying to know)).

In short: you need a fresh MacPorts (1.6, I had 1.3.something):

$ sudo port selfupdate

, and then, wipe your current libxml2 (along with any dependent libraries, MacPorts will tell you which):

$ sudo port uninstall libxml2@2.6.23_0

(You might not need the version number; I had tried to get the new libxml2 before this so there was an Activating libxml2 2.6.31_0 failed: Image error: Another version of this port thingy going on.) Then just:

$ sudo port install libxml2 libxslt

and you'll be ready to do the final (after removing any misbehaving lxml (compiled with the old libxml2) from site-packages) :

$ sudo easy_install lxml

But why?

Because lxml 2 is a marvellous thing for churning through XML and HTML with Python. There was always XPath and XSLT, C14N in lxml 1.x too (admittedly also in 4Suite as well; Python has had strong XML support for many, many years). But in lxml 2, you also get:

• CSS selectors
• much improved "bad HTML" support (including BeautifulSoup integration)
• a relaxed comparison mechanism for XML and HTML in doctests!

And more, I'm sure.

So, why all this Yak Shaving tonight? Just this afternoon I put together an lxml and httplib2-based "REST test" thing. Doesn't sound too exciting? I think it may be, since I use it with the old but still insanely modern doctest module, namely its support for executing plain text documents as full-fledged tests. This gives the possibility (from a tiny amount of code), to run plain text specifications for ROA-apps with a single command:

Set up namespaces:

 >>> xmlns("http://www.w3.org/1999/xhtml")
 >>> xmlns(a="http://www.w3.org/2005/Atom")

We have a feed:

 >>> http_get("/feed",
 ...     xpath("/a:feed/a:link[@rel='next-archive']"))
 ...
 200 OK
 Content-Type: application/atom+xml;feed
 [XPath 1 matched]

Retrieving a document entry with content-negotiation:

 >>> http_get("/publ/sfs/1999:175",
 ...         headers={"Accepts": "application/pdf"},
 ...         follow=True)
 ...
 303 See Other
 Location: /publ/sfs/1999:175/pdf,sv
 [Following..]
 200 OK
 Content-Type: application/pdf
 Content-Language: sv
 [Body]

That's the early design at least. Note that the above is the test. Doctest checks the actual output and complains if it differs. Declarative and simple. (And done before, of course.)

Ok, I was a bit overambitious with all the "ROA"; this is mostly for checking your content-negotiation, redirects and so on. But I think it'll be useful for a large system I'm putting together, which will (if things work out well) at its core have a well-behaved Atom-based resource service loaded with (linked) RDF payloads. Complete with feed archiving and tombstones, it will in turn be used as fodder for a SPAQRL service. (Seriously; it's almost "SOA Orchestration Done Right", if such a thing can be done.) But this I'll get back too..

(.. I may call the practice "COURT" — Crafting Organization Using Resources over Time. It's about using Atom Entries as manifests of web resources with multiple representations, fully described by RDF and working as an over-time serialized content repository update stream. (Well, that's basically what Atom is/works perfectly for, so I'm by no means inventing grand things here — grand as they IMHO are.)

A Great Day for Specificity

dbpedia.org - Using Wikipedia as a Web Database

[...] dbpedia can also be seen as a huge ontology that assigns URIs to plenty of concepts and backs these URIs with with dereferencable RDF descriptions.

We have advanced a tech level!

This is really good timing, as I just recently considered using TDB URNs for referencing non-retrievable things and concepts (using TDB-versions of URLs to Wikipedia articles and web sites). Finding out if the idea of TDB and DURI URNs have been long since abandoned was my next step down that path. (Then there's the use of owl:InverseFunctionalProperty and bnodes (or "just-in-time"-invented ad-hoc URIs) and throwing in owl:sameAs if official ones are discovered..)

With DBPedia, a lot of that becomes unnecessary. The availibility of public, durable URIs for common concepts will surely ease the integration of disparate sources of knowledge. That is, if we start to use dbpedia-URIs in our statements.

And gone will be the strangeness of saying "I'm interested in [the word 'semantics']", "This text was edited with [the website <http://vim.org>]" and "I was born in [the wikipedia article about 'Stockholm']"..

Knowledge, The Bits and Pieces

In a recent post by Lee Feigenbaum, he talks about Using RDF on the Web. Naturally I find this very interesting. In my work on the Oort toolkit, I use an approach of "removing dimensions": namespaces, I18N (optionally), RDF-specific distinctions (collections vs. multiple properties) and other forms of graph traversing. This is done with declarative programming very similar to ORM-tools in dynamic languages (mainly class declarations with attributes describing desired data selection). The resulting objects become simple value trees — with the added bonus of automatic JSON-serialization.

Ideally, this approach will be deterministically reversible. I have not yet implemented it, but the idea is that the declared classes (RdfQueries in Oort — let's call them "facets" here) could take JSON as input and reproduce triples. Using checksums of the JSON would make over-the-web editing possible.

Since the task of "updating" a subgraph is somewhat difficult at best, I think a basic "wipe and replace" approach may be simplest. There are many dangers here of course (removing a relation must not remove knowledge about its object — unless perhaps if that object is a bnode..).

Albeit all of this is Python to the core now, nothing in the design — declarative as it is — prevents the approach from being more general. Indeed, such facets, were they themselves serializable, could be used as structured content retrieval over-the-web too. Ok, maybe I'm reinventing SPARQL now.. Or should I use SPARQL for their remote execution? It seems reasonable (I mean that's exactly how ORMs do SQL).

Now, I seem to end up in the RPC/RESTful camp with this. A solution to that could be: use the facets on the client, having them use SPARQL for retrieval. Then you have clients working against any kind of SPARQL endpoint, mashup-style. Still, if facets are completely reversible, they may be powerful, aware tools, and perhaps an alternative to SPARQL in intercommunication? That's a pipe dream for me right now though.

The SPARQL-way is of course only for reading data. Fine, the RDF model may be too expressive (fine-grained) to be practical for direct over-the-web editing in specific situations anyway. A confined approach such as this JSON+checksums+"careful with nested knowledge" may be better for this.

I think of JSON-RPC here as I view e.g. microformats with GRDDL — it's leverage, not a final solution. RDF models are the ideal, we may just need reversible simplifications to gain mass usability. I touched upon JSON for integrated apps, but stuff like Atom for "talking to strangers" in response to a previous post by Lee. His post which I refer to above hints at better stuff than just Atom if we want RDF all the way also in more specific apps.

So, what I'm saying is really: I also desire the best of two worlds. The RDF model is sound and extremely valuable, but after all, simple domain-specific object manipulation is what makes the web go round. A solution may be some form of O/R mapping for RDF. The difference is not wanting to forget the data layer (there is no monster in the closet as there is with raw SQL..), just streamline some of the work with it.

Let's hope 2007 will be a good year for the knowledge revolution.

Think Outside The Box

I just realized it. The reason for the increasing popularity of dynamic languages may just be:

• Some years ago, many developers sat in front of humongous 19" CRTs and about 1600x1200 or whatnot.


• Nowadays, most of us sit with sleek notebooks with quite the lesser screen size and resolution.

Thus, it's much more of an annoyance that we have to scroll through rows and rows of boilerplate and wrapped lines of type declaration harnessing. It simply has to go.

(Nevermind productivity or the fact that these new tiny things pack enough CPU power to make DNA sequence processing a BLAST (pardon the pun).)