RFC: Add PythonInputSource to create py-based graphs

[joshmoore]

In order to manipulate a JSON-LD structure before creating a graph (specifically to simplify rdf:Seq handling), it is
currently necessary to use json.loads followed by dumps and then let Graph().parse() re-load. By detecting dict
instances and creating a PythonInputSource, a single call to loads suffices.

Proposed Changes

• add rdflib.parser.PythonInputSource
• return instances from create_input_source
• detect instances in rdflib.plugins.shares.jsonld.util.source_to_json

[nicholascar]

Hi @joshmoore thanks for looking out for more efficiencies here: we are really keen to improve rdflib's JSON-LD performance!

Two points:

1. can you please add PythonInputSource to parser.py's _all_variable?
2. can you please add an equivalency test - an assert - to your test? You produce both "Current workaround" & "Desired behavior" output which, I'm sure, are the same, but your test doesn't actually test for that, so automated test running won't see the equivalent behaviour

[joshmoore]

Thanks for the feedback, @nicholascar. Glad to hear there's interest. I'll tackle the above as well as put some docs into place and then ping you.

[joshmoore]

@nicholascar: changes pushed. Not sure why this failure appeared:

	nosetests: error: no such option: --with-timer

[nicholascar]

Not sure why this failure appeared:

Probably because between you submitting the original PR and the testing now we've changed the tests in RDFlib from nose to pytest, so something's not right there.

I'll have a look tomorrow to see if I can work it out.

BUT, I've also come up with another request for you, sorry! You say in your PythonInputSource:

"Constructs an RDFLib Parser InputSource from a Python data structure loaded from JSON with json.load or json.loads."

But the "from JSON" bit's not strictly necessary. I could load your PythonInputSource with a native Python dict, data from YAML or any other source I cared to parse into a Python object, you just happen to have used JSON. So I think the class documentation needs to be made more generic.

I could make this change but I just want to test the assumption with you first.

[joshmoore]

I could make this change but I just want to test the assumption with you first.

Happy to loosen the docs or to have you do them 👍 I just didn't want to oversell this since as you say, nothing else has been tested. 😉

[joshmoore]

Merged origin/master to hopefully fix the test and tried to make it clear that json is just one example of a way to construct the Python data.

[nicholascar]

Looks good, all my queries addressed!