Enable (and test) xml export of pure XBlocks

[cpennington]

[LMS-179]
[LMS-209]
[LMS-1345]

[cpennington]

@nedbat @sarina @antoviaque: Review?

[antoviaque]

@cpennington Did a few tests, importing/exporting the mentoring XBlocks.

What worked (thanks!):

• Creating a <mentoring> XBlock in Studio and exporting the course
• Adding children to the <mentoring> block in the XML export, and importing it back -- the children had been correctly created

What didn't work:

• Re-exporting the course once the children had been added to the block. Traceback: https://gist.github.com/antoviaque/8497633 . XML used: https://gist.github.com/antoviaque/8497644 Update: I've actually tried to re-export again later, and was able to get the XML, including the children - so that might not be what caused the original traceback

• Although I'm not sure if that's the real cause, importing XML with an XBlock which had children with children themselves, like https://gist.github.com/antoviaque/8497920 . Traceback obtained during the archive import: https://gist.github.com/antoviaque/8497860

Let me know if you need additional details/tests.

[cpennington]

Hrm... Thanks for checking that out. XBlocks have been deprioritized (pushed till next sprint) so that we can finish up some i18n work, but I'll try and get this XML stuff sorted out after that's done.

[antoviaque]

@cpennington Okidok - thanks!

[cpennington]

@nedbat, @sarina: Talked w/ @antoviaque, and we weren't able to reproduce one of his issues, and identified another as a bug in his code, rather than this code. Can you guys review, and we can see about merging this?

[sarina]

There's like no coverage of xml_exporter.py.

What's the best way to test this code out?

[cpennington]

cms/djangoapps/contentstore/tests/test_contentstore.py uses export_to_xml a lot. It just doesn't get reported as part of the coverage for common/lib stuff. @wedaly what would we need to do to fix that? It seems odd not to count that sort of coverage.

[nedbat]

Minor point of style: don't need the [], a generator expression would work also.

[cpennington]

@nedbat: Addressed your concerns. Have anymore?

[nedbat]

I'm all out. 👍 :)

[sarina]

LGTM 👍

[cpennington]

@sarina, @nedbat can you review the last few commits? They're to add some testing of the api wrapping.

[sarina]

put a comment here explaining what this is a dictionary of (most values are [{}] which is hard to understand why this is necessary)

[sarina]

2 tweaks and 2 questions, then 👍 by me

[nedbat]

If we get rid of the __test__ = False above, then we can also get rid of this here, I think.

[nedbat]

Figure out the best way to avoid __test__ if you can, then 👍