Problem understanding some unit tests in Python distribution

[André]

Hi everyone,

In the hope of perhaps contributing some additional unit tests for
Python (thus contributing back to the community), I dove in the code
and found something that should be simple but that I can not wrap my
head around.

In list_tests.py, one finds
===
from test import test_support, seq_tests

class CommonTest(seq_tests.CommonTest):

    def test_init(self):
        # Iterable arg is optional
        self.assertEqual(self.type2test([]), self.type2test())
# etc.
===

Wanting to figure out what the type2test() method does, I looked in
seq_tests.py and found the following:

===
class CommonTest(unittest.TestCase):
    # The type to be tested
    type2test = None

    def test_constructors(self):
        l0 = []
        l1 = [0]
        l2 = [0, 1]

        u = self.type2test()
        u0 = self.type2test(l0)
        u1 = self.type2test(l1)
        u2 = self.type2test(l2)

# etc.
===
No where do I find a definition for the type2test() method - other
than seeing it as a class variable defined to be None by default.  I
looked in unittest.TestCase and did not see it anywhere.

Am I missing something obvious?  I would appreciate if someone could
point me in the right direction.

Cheers,

André