Add support for date-time skeletons

[mbirtwell]

The skeletons for dates and times are described on
http://cldr.unicode.org/translation/date-time-patterns under
Additional Date-Time Formats. And are useful when you want to some more
control over formatting dates and times but don't want to force all
locales to use the same pattern.

[codecov-io]

Current coverage is 84.26%

Merging #265 into master will increase coverage by +0.05% as of 6c99718

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  Stmts         3770    3776     +6
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+ Hit           3175    3182     +7
  Partial          0       0       
+ Missed         595     594     -1

Review entire Coverage Diff as of 6c99718

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[etanol]

Interesting. However, dedicated test code is missing. I know, doctests should cover it but I have a feeling that, if not me, somebody else will ask you for it.

[xmo-odoo]

I found this while looking into intervals (as they use skeletons), this PR implements basic support for skeletons, but doesn't seem to support skeleton matching (see "matching skeletons" sub-section) and match adjustment, is that normal/intentional?

I also am not sure about the inheritance support: the spec states

dateFormatItems inherit from their parent locale, so the inherited items need to be considered when processing.

which I read as each skeleton (dateFormatItem/@id) not found on the current locale being looked up in the parent locale, is that automatically resolved by babel?

[sils]

@etanol uhm I'mnot quite sure I agree with you here, that would mean that we have redundant tests which is nothing but a maintenance overhead, am I wrong?

[etanol]

@sils1297 Yes, they are an overhead. However, if you check the test suite you will find that there are quite a few tests that are identical to the corresponding docstring. Since I don't know the whole history of Babel, I'm not clear on how this redundancy originated.

[sils]

@etanol so we probably want to avoid redundancy from now on and approve PRs that remove identical tests, agreed? Code duplication is bad and doctest is a great invention to reduce testing overhead, we should use that advantage IMO.

[mbirtwell]

@sils1297 I've removed the commit which added the extra tests from this PR.

@xmo-odoo I'll be honest I did the minimal here that I needed for my use-case. Which didn't include skeleton-matching although that looks nice I think this is still useful without that. Inheritance however seems to have been handled for me by babel's CLDR import mechanism. e.g. there is no 'hm' skeleton in the en_GB.xml file but the following works:

In [1]: import babel

In [2]: l = babel.Locale.parse('en_GB')

In [3]: l.datetime_skeletons['hm']
Out[3]: <DateTimePattern 'h:mm a'>

[mbirtwell]

In CLDR 26, which babel is currently using there is a partial list of skeletons that appears to get correctly merged with the parent locale (en_US I think in CLDR 26, although I believe that is also changing).

[xmo-odoo]

@mbirtwell ok, good to know then (I only checked 23.1 — used by the last release — and 28 — latest CLDR — and neither has "partial" skeletons lists)

[akx]

@mbirtwell Could you please rebase on top of 2.2.0? One doctest will also need updating based on the current CLDR data.

================================== FAILURES ===================================
_______________ [doctest] babel.core.Locale.datetime_skeletons ________________
EXAMPLE LOCATION UNKNOWN, not showing all tests of that example
??? >>> Locale('fr').datetime_skeletons['MEd']
Expected:
    <DateTimePattern u'E d/M'>
Got:
    <DateTimePattern 'E dd/MM'>

==================== 1 failed, 520 passed in 5.28 seconds =====================

[akx]

Thanks @mbirtwell! 🎉