Weird behavior of nosetest

In this article, I shared my experience with a weird bug in nosetests. Let ’s see what is it, and what we could do to fix/avoid it in future.

With unittest, two functions setUp and tearDown will call before and after every test case function. With code bellow, I expected test cases will be passed:

import unittest

class TestSuite(unittest.TestCase):
    b = []

    def setUp(self):
        self.b.extend([10, 20])

    def tearDown(self):
        self.b = []

    def test_case_1(self):
        assert len(self.b) == 3
        assert self.b == [10, 20, 30]

    def test_case_2(self):
        assert len(self.b) == 3
        assert self.b == [10, 20, 40]

But when I run it with nose, the result is unexpected:

$> nosetest
FAIL: test_case_2 (test_module2.TestSuite)
Traceback (most recent call last):
File "/Users/knt/", line 19, in test_case_2
    assert len(self.b) == 3

Ran 2 tests in 0.001s

FAILED (failures=1)

What ’s happend ??? I expect after run test_case_1, tearDown will be called, so self.b is []. With next test case test_case_2, setUp run and self.b is [10, 20]. But in fact, at setUp value of self.b is [10, 20, 30]. I don’t know why??? I think there must be some problems with statement self.b = []. Anything related pointer, I guess. I still didn’t figure it out, but I find a way to work around this bug. Just change self.b = [] to del self.b[:]:

    def tearDown(self):
        del self.b[:]

Now, lunch time !!!


I post my question on stackoverflow, and the answer explains about this bug.

In tearDown function, when I self.b = [], the b variable is not class variable. In fact, python will create a new instance variable for this class. After that, at next test case, nose and unittest will create new instance of TestSuite so, self.b of this instance still is a class variable. The reason why del self.b[:] works is it does not create a new instance variable, but still access class variable.

This bug gives me a new lesson when dealing with class variables. The “best practice” with me is every time I want to access class variable in method, I should use self.__class__.<variable_name>.

Kien Nguyen Trung

A father, husband and Elixir lover.

Ho Chi Minh, Vietnam