Add test to avoid unwanted dependencies among backends

Ibis had some interdependencies among backends that have been causing some issues, besides making the codebase unnecessarily complicated.

We are finishing a major refactoring to make the dependencies more reasonable. Mainly, to avoid unrelated backends to depend among them (bigquery or spark depending on impala), and creating some base backends with the code shared among them.

I think it would be great if we can have a test that makes sure there are not unwanted dependencies among backends.

I think we can probably have a list of the backends with the known dependencies, load each backend, and check if any unknown backend has been loaded. As a general idea (code not expected to work):

BACKEND_DEPS = {'sqlite': ['base_sqlalchemy'],
                'bigquery': ['base_sql'],
                ...}

for backend, known_deps in BACKEND_DEPS:
    importlib.load(f'ibis.backends.{backend}')
    assert not any(mod in sys.modules if mod.startswith('ibis.backends') and mod not in known_deps)
    importlib.unload(...)

[datapythonista]

If del sys.modules... works I’d go for that. Adding tests for each backends is very repetitive, and we’ll end up missing backends in the future.

[datapythonista]

Thanks @matthewmturner that sounds like good ideas. But I don’t think we can do number 2. If you move the conftest.py file, I think it won’t be applied to the backend tests then, which is it’s goal.

This is a nice to have, but surely not a blocker, and lower priority than the refactoring itself, but if you feel like working on it, it’ll be useful to know when we’re done with the refactoring. 😃