Merge lp://staging/~fwereade/pyjuju/restrict-relation-broken-context into lp://staging/pyjuju

Nice work, some minors.

[0]

Cache instance attributes could always use a documentation line on
what's being cached.

hook.py line 269

self._relation_cache = None

[1]

set_value/set

        return fail(NotImplementedError(
            "Cannot change settings in broken relation"))

That doesn't seem like the appropriate error in this
context. Something more specific to the problem at hand seems
appropriate, ie. RelationBrokenContextError

[2] pep8 nags, trailing newline at EOF, over 80 (unrelated to diff) in hook.py

unit_state = yield service_state_manager.get_unit_state(self._unit_name)

[3]

seems like the mixin here should be merged into hookcontexttestbase,
all the tests have the same pair of base classes.

HookContextTest(HookContextTestBase, CommonHookContextTestsMixin):

Thanks Kapil.

[0]

Good point, thanks.

[1]

Fair enough :).

[2]

I'll take a look at those.

[3]

I'd prefer to avoid that: (1) the mixin depends on .service and .context, which are not part of the TestBase, and (2) they do have different responsibilities, and it feels somewhat coincidental that all the subclasses currently happen to use both base classes' services.

(Ha, "over 80" -- I thought the whole file was riddled with pep8 violations, and was bemused when it only reported 2 :))

LGTM +1