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A common thing to do in setUp or test* methods is to replace some module attribute with something else, either to mock an object calling an external resource or to test platform-specific behavior (for example, changing os.name before calling some function). Care has to be taken to restore the initial object with addCleanup, tearDown or in a finally block.
I propose that a new method TestCase.patch (inspired by mock.patch, but more limited in scope) be added, to allow such usages (each example is standalone):
 def setUp(self):
 self.patch(socket, 'socket', MockSocket)
 def test_default_format(self):
 self.patch(os, 'name', 'posix')
 self.assertEqual(get_default_format(), '.tar.gz')
 self.path(os, 'name', 'nt')
 self.assertEqual(get_default_format(), '.zip')
 def setUp(self):
 self.patch(sys, 'path', sys.path.copy())
In each example, patch(object, attribute, value) does this: save object.attribute, set object.attribute to value, register a cleanup function to restore object.attribute.
I assigned to Michael so that he can kill this idea early if he has reason to do so. If not, please move stage to "patch needed" (no pun). I am willing to work on a patch for 3.3 and unittest2 (not sure which is first :)