librdf (python bindings): Can I preserve blank node IDs on parsing?

Question

I'd like to write a Python program that:

Queries an openrdf-sesame database through the REST interface
Parses the result into a local graph with librdf
Performs some manipulations that result in add and delete statements
Executes those results back against the openrdf-sesame REST interface

The problem I'm having: librdf assigns its own blank identifiers to the blank nodes as the source graph is parsed, so if one of the steps in my transaction (of #4, above) is to delete a blank node, this will fail -- because my local blank node identifier is different from the remote server's blank node identifier.

How can I tackle this? Is there a way to ask librdf to preserve blank node identifiers on parsing a graph? Or some other approach?

Thanks,

-B

Edit: The core issue

Based on Signified's suggestions, here (I suppose) is the crux of my problem: librdf ignores even explicit nodeIDs from the source graph:

>>> import RDF
>>> m = RDF.Model()
>>> p = RDF.Parser()

>>> s = """<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
  <rdf:Description rdf:nodeID="one_node">
        <rdf:type rdf:resource="http://my.type"/>
  </rdf:Description>
</rdf:RDF>
"""
>>> p.parse_string_into_model(m,s,"http://base_uri")
>>> ss = m.find_statements(RDF.Statement(None,None,None))
>>> list(ss)[0].subject.blank_identifier
'r1298398951r3374r3'

Same thing with NTriples:

>>> s = """_:my_node_name <is>; <okay>;.\n""" 
>>> p = RDF.NTriplesParser() 
>>> p.parse_string_into_model(m,s,"base_uri";) 
>>> ss = m.find_statements(RDF.Statement(None,None,None)) 
>>> list(ss)[0].subject.blank_identifier
 'r1298406586r15211r1'

Accepted Answer

Since you speak of "graphs", I assume that your queries are CONSTUCT/DESCRIBE?

Labelling of blank-nodes

Are the results from the REST interface in RDF/XML? If so, unless there's rdf:nodeID attributes assigned, there are no obvious labels to assign to a blank-node. Even with rdf:nodeID values, parsers are not required to preserve these as blank-node labels, and may (varyingly) append them onto prefixes, or add counters, etc.

For RDF/XML blank-nodes without rdf:nodeID, parsers typically have a prefix like bnode or genid and append a counter _:bnode0, _:bnode1 etc. There's no agreed upon rule-of-thumb on how this should be done (and different parsers might encounter bnodes in different order).

So, you shouldn't expect consistent blank-node labelling across different parsers. Similarly, I wouldn't imagine that the bnode labelling of librdf for RDF/XML parsing is configurable.

Solutions?

An easy solution might be to try return results in N-Triples where blank-nodes are explicitly named. (I think that it's a safe assumption that librdf will keep the native labels for blank-nodes.)

If not possible (tricky)...

One option might be to not specify the blank node in the remove statement request. This will work fine if the remaining constants uniquely identify a particular triple. Otherwise, you can query Sesame with the blank-nodes as variables: this will return a set of candidate statements to remove. You can then try to figure out which candidate statement you want to delete (maybe using a simple entailment check :O).

librdf (python bindings): Can I preserve blank node IDs on parsing?

Edit: The core issue

Same thing with NTriples:

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