[issue17689] Fix test discovery for test_tarfile.py

[Ezio Melotti]

Ezio Melotti added the comment:

> I found requires_bz2 and requires_lzma that already exist;

There's also requires_zlib.  import_module is also somewhat similar, but it's used to skip the whole test file when the module is missing.

> Here's another thought; would it be more useful to have a general
> version of this skip decorator in test.support

We were discussing this a couple of days ago on #python-dev.  Adding a skip_unless_module('modulename') (or requires_module('modulename')) and get rid of the several requires_* is certainly an option.

skip_unless_* (or requires_*) is shorter and more readable than skip_unless_module('modulename'), so this would be my preferred choice if it's defined and used within a single test module.  However if we add something to test.support, I'd rather have a more generic skip_unless_module('modulename') and get rid of the requires_*.
It's should also be possible to define specific ``skip_unless_x = skip_unless_module('x')`` in the test modules if necessary.

> should I just add a requires_gzip to test.support and use those three
> in test_tarfile?

I think that's reasonable for now -- we can always refactor it later and replace them with a skip_unless_module()

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<http://bugs.python.org/issue17689>
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