Persist variable changes between tests in unittest?

As some comments have echoed, structuring your tests in this manner is probably a design flaw in the tests themselves and you should consider restructuring them. However, if you want to do this and rely on the fact that the test runner you are using executes them in an alphabetical (seemingly) order then I suggest the following.

Similar to what @Matthias was saying but I would do one thing differently for the cases where you may decide to inherit from the class at a later date.

from unittest import TestCase, main as unittest_main


class TestSimpleFoo(TestCase):
    foo = 'bar'

    def setUp(self):
        pass

    def test_a(self):
        self.assertEqual(self.__class__.foo, 'bar')
        self.__class__.foo = 'can'

    def test_f(self):
        self.assertEqual(self.__class__.foo, 'can')


if __name__ == '__main__':
    unittest_main()

The difference between this answer and @Matthias’s answer you accepted is the explicit declaration of the class versus the lookup of said class reference.

TestSimpleFoo vs self.__class__

I prefer the dynamicness so I can inherit the tests later and run both test classes back to back and not have any cross over between the two. Because if you would choose to inherit from this class, explicitly naming the class reference would cause both test classes to run against that reference rather than their own respective classes.