Fix #351: batch updates via context manager

[tseaver]

See #351.

Add 'batch' property to '_PropertyMixin'.

• Defers / aggregates repeated calls to '_patch_properties' within the scope of a 'with foo.batch:' block.
• Issues a single '_patch_properties' call with the aggregated dict, IFF the block exits without an exception.
• If an exception is raised from the with block, all changes are discarded.

E.g:

with key.batch:
    key.content_type = 'image/jpeg'
    key.content_disposition = 'attachment; filename="photo.jpg"'

results in a single PATCH request, with payload:

{'contentType': 'image/jpeg', 'contentDisposition': 'attachment; filename="photo.jpg"'}

[dhermes]

This is great!

LGTM except for the Travis lint failure.

Ping me after fixing so I can do a final pass.

[tseaver]

The Travis failure is actually a bug in the "optimize lint" code: because not all files are considered, the 'abstract-class-not-used' warning kicks in. I can "fix" the failure by checking in a spurious change to bucket.py or key.py.

[dhermes]

Aha! That seems like the easiest get-out-of-jail free card for this PR.

Any ideas for how to fix this going forward? (I know its my hack, but no good ideas come to mind quickly.)

Seems unlikely enough that maybe we just don't waste engineering effort on trying to fix this corner case?

[tseaver]

I'd be tempted to just disable the pylint abstract-class-not-used and abstract-class-little-used checks, myself.

[dhermes]

They are helpful in finding dead code. IMO, the more "out-of-the-box" our usage of pylint the better.

[dhermes]

@tseaver Green Travis, feel free to merge.

[coveralls]

Changes Unknown when pulling 245b0e9 on tseaver:351-batch_updates_via_context_manager into * on GoogleCloudPlatform:master*.

[tseaver]

I find it over-opinionated, myself.