Are unittest attributes mutable in Python?

Question

Suppose I have this integration test

class TestClass(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        cls.key = '123'

    def test_01_create_acc(self):
       user = create_account(...)
       self.key = user.key

    def test_02_check_account(self):
       user = check_account(..)
       self.assertEqual(self.key, user.key)

It looks like the attribute self.key is not mutable. It stays with the old value from setUpClass. But isn't setUpClass only called once?

The account function creates a key randomly for security reason, so I am not allowed to pass in my secret key. It returns the key, so I need to modify that attribute. Can I?

It looks like each test_ case is isolated.

my_gloabl = None

def setUpClass(cls):
    cls.key = my_global

If I change my_global in test1, test2 will get None.

David Robinson · Accepted Answer · 2012-10-16 18:44:03Z

1

The class is set up only once. But each test method is actually called from a different instance of that test.

You can demonstrate this using the id function, which will return a different number for each object:

import unittest

class TestClass(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        print "setup"

    def test_01_create_acc(self):
        print id(self)

    def test_02_check_account(self):
        print id(self)

unittest.main()

On my computer, this printed:

setup
4300479824
.4300479888

Note how the setup method was called only once, but the id of the instance for test1 and test2 are different.

answered Oct 16, 2012 at 18:44

David Robinson

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3 Comments

User007 Over a year ago

Thanks. So you are saying each test is indeed isolated. If self is different, doesn't it mean this is a fork?

Bakuriu Over a year ago

No, fork does not anything to do with this. It's just that when you build a suite(or when unittest builds it) it does something like suite=Suite(MyTestCase('test_name'), MyTestCase('test_name2'),...) thus it creates more instances of the class. Avoid dependencies between tests. They are usually unneeded and make everything harder.

David Robinson Over a year ago

Exactly- multiple instances of the same class (all in the same process, not anything like a fork). Also note that there is no guarantee what order tests will run it, another reason to avoid dependencies between them.

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