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If the COLUMNS environment variable is set to a value other than 80 then test_argparse.py yields 80 failures. The value of the COLUMNS environment variable affects line wrapping of the help output and the test cases assume line wraps at 80 columns. So setting COLUMNS=160, for example, then running the tests will reproduce the failures. The fix is to invoke: os.environ["COLUMNS"] = "80".
A proposed patch for py3k/Lib/test/test_argparse.py is attached (test_argparse.py.COLUMNS.patch)

Shouldn't the tests calculate line wrapping based on what is set, rather than brute forcing it to be 80?

The best solution would be to make sure that a few different column widths are tested. However, in the meantime, the tests do assume 80 columns, so I think it's correct to specify that using os.environ as suggested.
One problem with the proposed patch is that it makes the change globally, and we should be restoring the original setting after the end of the argparse tests.

There's a handy utility for this in test.support: EnvironmentVarGuard.

I agree with Steven: for the current tests we should specify (and restore) 80 columns. We might want to add additional tests at different column widths.

That is a very good point, bethard, that setting os.environ["COLUMNS"] in my suggested patch (test_argparse.py.COLUMNS.patch) is global and should be test-local. I've attached an updated patch (test_argparse.py.COLUMNS.update1.patch) which uses setUp() and tearDown() to prepare and restore the COLUMNS environment variable. The one difference from my previous patch is that instead of setting the COLUMNS environment variable to 80 I just unset it.
I also considered EnvironmentVarGuard, as suggested by r.david.murray, but I'm not sure it's designed for global setting of environment variables. EnvironmentVarGuard appears to have been designed to be used as a context manager for an individual test, but the COLUMNS environment variable needs to be adjusted for *every* test.

Your code is fine (though to my tastes a bit verbose...if it were me I'd just put the code in the setUp and tearDown methods and hardcode 'COLUMNS' (it isn't like the name COLUMNS is going to change)...but that's just personal style).
The EnviormentVarGuard version would look like this (untested):
 def setUp(self):
 self.guard = EnvironmentVarGuard()
 self.environ = self.guard.__enter__()
 # Current tests expect 80 column terminal width.
 self.environ['COLUMNS'] = 80
 def tearDown(self):
 self.guard.__exit__(None, None, None)
You could of course delete COLUMNS as you did, but I thought setting it to 80 would be more explicit.
Another comment about the patch: by inspection it appears that adding setUp and tearDown to TestCase isn't enough, since subclasses and mixins define those without calling the superclass versions.

Thanks for the input, r.david.murray. I've updated my patch and attached it to take into consideration your comments: test_argparse.py.COLUMNS.update2.patch. The updated patch uses EnviormentVarGuard as suggested, except that it slightly tweaks EnviormentVarGuard so the context manager protocol methods don't have to be invoked directly.
It was also pointed out that "adding setUp and tearDown to TestCase isn't enough, since subclasses and mixins define those without calling the superclass versions", which is true. However, the tests that override setUp() happen to be those that don't depend on the COLUMNS environment variable.

I don't think it is worthwhile to jump through hoops to avoid calling the special methods. Your patch also creates an unnecessary dependency on the internal implementation of EnvironmentVarGuard (ie: the fact that currently __enter__ happens to return self...this is *not* part of EnvironmentVarGuard's interface contract).

As noted in issue10235, this is responsible for buildbot failures:
http://www.python.org/dev/buildbot/all/builders/PPC%20Leopard%203.x/builds/685/steps/test/logs/stdio 

Fixed with a variant of Denver's last patch in r86080 for 3.X and r86083 for 2.7.