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An RDFa parser wth a simple dictionary-like interface.

Project description

Date: 2007-03-08 13:27:25 -0500 (Thu, 08 Mar 2007)
Version: 5519
Author: Nathan R. Yergler <nathan@creativecommons.org>
Organization: Creative Commons
Copyright: 2006, Nathan R. Yergler, Creative Commons; licensed to the public under the MIT license.

Installation

rdfadict and its dependencies may be installed using easy_install (recommended)

$ easy_install rdfadict

or by using the standard distutils setup.py:

$ python setup.py install

If installing using setup.py, lxml will also need to be installed.

Usage

Document Purpose

This document is intended to provide a set of literate tests for the rdfadict package; it is not intended to provide thorough coverage of RDFa syntax or semantics. See the RDF Primer or the RDFa Syntax for details on RDFa.

rdfadict parses RDFa metadata encoded in HTML or XHTML documents. It can parse a block of text (as a string), or a URL. For example, given the following block of sample text:

>>> rdfa_sample = """
... <div xmlns:dc="http://purl.org/dc/elements/1.1/"
...      xmlns:xsd="http://www.w3.org/2001/XMLSchema">
... <h1 property="dc:title">Vacation in the South of France</h1>
... <h2>created
... by <span property="dc:creator">Mark Birbeck</span>
... on <span property="dc:date" type="xsd:date"
...          content="2006-01-02">
...   January 2nd, 2006
...    </span>
... </h2>
... </div>"""

Triples can be extracted using rdfadict:

>>> import rdfadict
>>> base_uri = "http://example.com/rdfadict/"
>>> parser = rdfadict.RdfaParser()
>>> triples = parser.parsestring(rdfa_sample, base_uri)

We define the variable base_uri to let the parser know what URI assertions without subjects apply to.

Based on our example text, we expect to get three triples back – title, creator and date. Triple are indexed as a dictionary, first by subject, then by predicate, finally retuning a list of objects. For example, a list of all subjects is retrieved using:

>>> triples.keys()
['http://example.com/rdfadict/']

If assertions were made about resources other than the default, those URIs would appear in this list. We can verify how many predicates were found for this subject by accessing the next level of the dictionary:

>>> len(triples['http://example.com/rdfadict/'].keys())
3

Finally, we can retrieve the value for the title by fully dereferencing the dictionary:

>>> triples['http://example.com/rdfadict/'][
...     'http://purl.org/dc/elements/1.1/title']
['Vacation in the South of France']

Note that the objects are stored as a list by the default triple sink.

Multiple Assertions

Because the property attribute always denotes triple with a literal string as its object and rel and rev denote triples with URIs as their objects, it is possible to make multiple assertions with a single HTML tag.

For example:

>>> multi_rdfa = """
... <div xmlns:foaf="http://xmlns.com/foaf/0.1/"
...      xmlns:dc="http://purl.org/dc/elements/1.1/">
...   This photo was taken by <a about="photo1.jpg" property="dc:title"
...   content="Portrait of Mark" rel="dc:creator"
...   rev="foaf:img"
...   href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.
... </div>
... """

In this statement we are making three assertions: two involving URI objects (specified by rel and rev), and one involving the property.

>>> import rdfadict
>>> parser = rdfadict.RdfaParser()
>>> multi_base_uri = "http://example.com/multiassert/"
>>> triples = parser.parsestring(multi_rdfa, multi_base_uri)

We expect the triples generated to have two subjects: the photo URI (for the rel and property assertions) and the href URI (for the rev assertion).

>>> len(triples.keys()) == 2
True
>>> 'http://example.com/multiassert/photo1.jpg' in triples.keys()
True
>>> 'http://www.blogger.com/profile/1109404' in triples.keys()
True

Finally, we verify that the assertions made about each subject are correct:

>>> len(triples['http://example.com/multiassert/photo1.jpg'].keys()) == 2
True
>>> triples['http://example.com/multiassert/photo1.jpg'] \
...          ['http://purl.org/dc/elements/1.1/creator']
['http://www.blogger.com/profile/1109404']
>>> triples['http://example.com/multiassert/photo1.jpg'] \
...          ['http://purl.org/dc/elements/1.1/title']
['Portrait of Mark']
>>> triples['http://www.blogger.com/profile/1109404']
{'http://xmlns.com/foaf/0.1/img': ['http://example.com/multiassert/photo1.jpg']}

Resolving Statements

When resolving statements, the REL, REV, CLASS and PROPERTY attributes expect a CURIE, while the HREF property expects a URI. When resolving CURIEs, un-namespaced values which are not HTML reserved words (such as license) are ignored to prevent “triple bloat”.

Given an example:

>>> rdfa_sample2 = """
... <div xmlns:dc="http://purl.org/dc/elements/1.1/"
...      xmlns:xsd="http://www.w3.org/2001/XMLSchema">
... <h1 property="dc:title">Vacation in the South of France</h1>
... <h2>created
... by <span property="dc:creator">Mark Birbeck</span>
... on <span property="dc:date" type="xsd:date"
...          content="2006-01-02">
...   January 2nd, 2006
...    </span>
... </h2>
... <img src="/myphoto.jpg" class="photo" />
... (<a href="http://creativecommons.org/licenses/by/3.0/" rel="license"
...    about="/myphoto.jpg">CC License</a>)
... </div>"""

We can extract RDFa triples from it:

>>> parser = rdfadict.RdfaParser()
>>> base_uri2 = "http://example.com/rdfadict/sample2"
>>> triples = parser.parsestring(rdfa_sample2, base_uri2)

This block of RDFa includes a license statement about another document, the photo:

>>> len(triples["http://example.com/myphoto.jpg"])
1
>>> triples["http://example.com/myphoto.jpg"].keys()
['http://www.w3.org/1999/xhtml#license']
>>> triples["http://example.com/myphoto.jpg"] \
...    ['http://www.w3.org/1999/xhtml#license']
['http://creativecommons.org/licenses/by/3.0/']

There are two things to note with respect to this example. First, the relative URI for the photo is resolved with respect to the base_uri value. Second, the “class” attribute is not processed, because it’s value is not in a declared namespace:

>>> 'photo' in [ n.lower() for n in
...      triples['http://example.com/rdfadict/sample2'].keys() ]
False

See the RDFa Primer for more RDFa examples.

Triple Sinks

rdfadict uses a simple interface (the triple sink) to pass RDF triples extracted back to some storage mechanism. A class which acts as a triple sink only needs to define a single method, triple. For example:

class StdOutTripleSink(object):
    """A triple sink which prints out the triples as they are received."""

    def triple(self, subject, predicate, object):
        """Process the given triple."""

        print subject, predicate, object

The default triple sink models the triples as a nested dictionary, as described above. Also included with the package is a list triple sink, which stores the triples as a list of 3-tuples. To use a different sink, pass an instance in as the sink parameter to either parse method. For example:

>>> parser = rdfadict.RdfaParser()
>>> list_sink = rdfadict.sink.SimpleTripleSink()
>>> parser.parsestring(rdfa_sample, base_uri, sink=list_sink)
[('http://example.com/rdfadict/', 'http://purl.org/dc/elements/1.1/title', 'Vacation in the South of France'), ('http://example.com/rdfadict/', 'http://purl.org/dc/elements/1.1/creator', 'Mark Birbeck'), ('http://example.com/rdfadict/', 'http://purl.org/dc/elements/1.1/date', '2006-01-02')]
>>> len(list_sink)
3

Note that the parse method returns the sink used. Since the sink we’re using is really just a list, the interpreter prints the contents upon return.

Limitations and Known Issues

rdfadict currently does not implement the following areas properly; numbers in parenthesis refer to the section number in the RDFa Syntax Document.

  • xml:base is not respected (2.3)
  • Typing is not implemented; this includes implicit XMLLiteral typing as well as explicit types specified by the datatype attribute (5.1)
  • Blank nodes are not guaranteed to work per the syntax document (5.2); if you try to use them, you will probably be disappointed.
  • Reification is not implemented (5.3).

Change History

0.3 (2007-03-08)

  • Fixed resolution of URIs v. CURIEs
  • Drop assertions with non-namespaced CURIEs as the predicate (per updated spec)
  • Updated test suite to comply with updated RDFa specification
  • Corrected subject resolution behavior for <link> and <meta> elements
  • Implemented entry point and extractor interface for compatibility with the ccrdf.rdfextract library.
  • Fixed parsing of rev assertions, which was formerly completely broken.

0.2 (2006-11-21)

  • Directly subclass list and dict for our sample triple sinks
  • Additional package metadata for PyPI
  • Additional documentation of sink interface and tests for the SimpleTripleSink

0.1 (2006-11-20)

  • Initial public release

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