self.attr resets between tests in unittest.TestCase

Question

I'd like to use a self.attr of a unittest.TestCase class, however it seems it is not persistent between tests:

import unittest

class TestNightlife(unittest.TestCase):
    _my_param = 0

    def test_a(self):
        print 'test A = %d' % self._my_param
        self._my_param = 1

    def test_b(self):
        print 'test B = %d' % self._my_param
        self._my_param = 2

if __name__ == "__main__":
    unittest.main()

This gives the following output:

test A = 0
test B = 0

Does the instance of unittest.TestCase change between test runs? Why?

Answer 1

It works that way because unittest.main() create separate object for every test (in this case two objects are created).

About your motivation: test should not change global state. You should revert global state to state before test in tearDown or test itself. It's very problematic if tests are changing global state, you will fall in scenarios you can't predict sooner or later.

import unittest

class TestNightlife(unittest.TestCase):
    _my_param = 0

    def test_a(self):
        print 'object id: %d' % id(self)
        print 'test A = %d' % self._my_param
        self._my_param = 1

    def test_b(self):
        print 'object id: %d' % id(self)
        print 'test B = %d' % self._my_param
        self._my_param = 2

if __name__ == "__main__":
    unittest.main()

output:

object id: 10969360
test A = 0
.object id: 10969424
test B = 0
.
----------------------------------------------------------------------
Ran 2 tests in 0.000s

OK