Adding gcloud.bigtable package.

[dhermes]

Initial commit just adds the basic import time check for gRPC and testing for all possible install states. It also adds a shadowed _testing/grpc that we can use to mock the installed library for tests.

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We can discuss alternatives to mocking out grpc imports in dhermes/gcloud-python-bigtable#10 or right here. As far as I know, there is no way to globally (i.e. between / before modules) step in front of __import__ without doing some serious engineering.

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@nathanielmanistaatgoogle Any better ideas than this one for allowing unit tests that "depend" on grpc to run? (None of the gRPC code is ever called in unit tests, just in system tests. Though the gRPC objects get mocked out, the imports need to work so nose doesn't get angry.)

[nathanielmanistaatgoogle]

I'm having a hard time finding my way around this - what problem are we solving?

[dhermes]

Point of this PR: Add the basic gcloud.bigtable subpackage. (No functionality yet, but will be bringing it over piecemeal from https://github.com/dhermes/gcloud-python-bigtable / http://gcloud-python-bigtable.readthedocs.org/en/latest/)

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Extra helpful in this PR: In gcloud.bigtable package __init__.py, add a helpful error message if grpc is installed but the gRPC core system libs are not on the LD_LIBRARY_PATH.

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Problem to solve (thing I asked Nathaniel about): Importing gRPC in unit tests (in a virtualenv created by tox) without actually installing https://pypi.python.org/pypi/grpcio or grpc core.

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Dan's solution to above problem: Make a fake grpc package and add it to the PYTHONPATH in the test environment (thanks to setenv, a tox feature)

[dhermes]

@tseaver Any thoughts?

@nathanielmanistaatgoogle Does my last comment clarify the purpose?

[nathanielmanistaatgoogle]

It helps! I'm still not completely clear though - why bend over backwards to function in an environment in which "the gRPC core system libs are not on the LD_LIBRARY_PATH"? If you're testing that gcloud-python is still usable-at-least-in-part in such a configuration, why not catch just ImportErrors where they come up and verify the limited functionality to be verified?

What's the relationship between gcloud-python and grpcio? Full dependency, or weird kind-of dependency, or something else?

I'm not saying anything's wrong here; I just don't yet understand.

[dhermes]

We don't want users of gcloud.datastore to have to deal with the requirements of gcloud.bigtable. Thus grpcio will not be required and we want a way to gracefully instruct users of gcloud.bigtable to install grpcio.

This is really just a stop-gap until users can reliably install gRPC core and grpcio.

[nathanielmanistaatgoogle]

Okay. Then it's crazy, but once one has bought into its particular craziness, there's nothing wrong with it. Since it's a stop-gap I counsel filing a clean-up bug and including at least one TODO(issue XXX): remove this temporary artifice in the code.

[dhermes]

RE: "crazy". I am very open to other ideas, if you have any.

This just seemed like the only way to rely on an "upstream" dependency without hurting users that didn't need / want it.

[dhermes]

@tseaver WDYT here?

[nathanielmanistaatgoogle]

The only other idea I have is "catch a raised ImportError", with the thought that that is what I think I've heard is "usually" done in these kinds of situations. Why is that the wrong thing in this case?

[dhermes]

@nathanielmanistaatgoogle "catch a raised ImportError" is exactly what we do, though doing it in unit tests would require digging into the guts of nosetests and likely making our own custom nose test runner() (i.e. writing code instead of scripting). This is not a path I wish to go down.

[nathanielmanistaatgoogle]

Thanks for saying that; I think I get it now.

[dhermes]

@tseaver PTAL. I dropped the test___init__.py module altogether, since the package is just an import and a pragma no cover. I also using print(..., file=sys.stderr) instead of raising a different exception.

The _testing/ directory and the use of it in tox.ini are the only other remaining changes.

[tseaver]

LGTM