Lib/test/test_difflib.py

import difflib
from test import test_support
import unittest
import doctest

class TestSFbugs(unittest.TestCase):

    def test_ratio_for_null_seqn(self):
        # Check clearing of SF bug 763023
        s = difflib.SequenceMatcher(None, [], [])
        self.assertEqual(s.ratio(), 1)
        self.assertEqual(s.quick_ratio(), 1)
        self.assertEqual(s.real_quick_ratio(), 1)

Doctests = doctest.DocTestSuite(difflib)

test_support.run_unittest(TestSFbugs, Doctests)