The ReadTest.test_readline test in Lib/test/test_codecs.py has two loops on "\n \r\n \r \u2028".split(). But as far as "\n \r\n \r \u2028".split() is empty list (because newline characters are whitespace characters), the bodies of these loops are never executed.

After fixing this bug, a number of other tests were exposed. I tried to fix them. The purpose of these tests is not entirely clear, so I'm not sure that it is properly grasped the idea of the author.

Test was added in issue1076985 and modified in issue1175396.

"\n \r\n \r \u2028".split() should have been "\n \r\n \r \u2028".split(" "), i.e. a list of different line ends.

> The purpose of these tests is not entirely clear, so I'm not sure that it is properly grasped the idea of the author.

I wrote the tests nearly 10 years ago, so it's no longer entirely clear to me either! ;)

Anyway, here's a patch that fixes the resulting test failures.

> "\n \r\n \r \u2028".split() should have been "\n \r\n \r \u2028".split(" "),
> i.e. a list of different line ends.

Yes, this is obvious fix, but explicit tuple of line ends looks more clear and 
error-proof to me.

True, here's an updated patch.

New changeset 305b6a5da852 by Serhiy Storchaka in branch '2.7':
Issue #20520: Fixed readline test in test_codecs.
http://hg.python.org/cpython/rev/305b6a5da852

New changeset 82d374a9bbc7 by Serhiy Storchaka in branch '3.3':
Issue #20520: Fixed readline test in test_codecs.
http://hg.python.org/cpython/rev/82d374a9bbc7

New changeset 1457fa0da200 by Serhiy Storchaka in branch 'default':
Issue #20520: Fixed readline test in test_codecs.
http://hg.python.org/cpython/rev/1457fa0da200

The change has broken the builds, for example, http://buildbot.python.org/all/builders/AMD64%20Snow%20Leop%203.x/builds/1197.

[ 29/389] test_codecs
Assertion failed: (maxchar >= 0x100), function _PyUnicode_CheckConsistency, file Objects/unicodeobject.c, line 410.
Fatal Python error: Aborted

Current thread 0x00007fff71296cc0 (most recent call first):
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/codecs.py", line 494 in read
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/codecs.py", line 548 in readline
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/test_codecs.py", line 169 in test_readline
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/case.py", line 574 in run
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/case.py", line 622 in __call__
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 125 in run
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 87 in __call__
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 125 in run
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 87 in __call__
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 125 in run
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/suite.py", line 87 in __call__
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/unittest/runner.py", line 168 in run
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/support/__init__.py", line 1685 in _run_suite
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/support/__init__.py", line 1719 in run_unittest
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1277 in <lambda>
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1278 in runtest_inner
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 967 in runtest
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 532 in main
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1565 in main_in_temp_cwd
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1590 in <module>
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 86 in _run_code
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 171 in _run_module_as_main
Traceback (most recent call last):
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 171, in _run_module_as_main
    "__main__", mod_spec)
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/__main__.py", line 3, in <module>
    regrtest.main_in_temp_cwd()
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 1565, in main_in_temp_cwd
    main()
  File "/Users/buildbot/buildarea/3.x.murray-snowleopard/build/Lib/test/regrtest.py", line 738, in main
    raise Exception("Child error on {}: {}".format(test, result[1]))
Exception: Child error on test_codecs: Exit code -6
make: *** [buildbottest] Error 1

Aha! It looks as a bug in UTF-7 codec.

The culprit:

diff -r b4e99bec0c8a Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py	Fri Feb 07 10:10:55 2014 +0200
+++ b/Lib/test/test_codecs.py	Fri Feb 07 17:14:12 2014 +0800
@@ -164,6 +164,8 @@
                 s = 10*(size*"a" + lineend + "xxx\n")
                 reader = getreader(s)
                 for i in range(10):
+                    if size==64 and self.encoding == 'utf-7' and lineend=='\u2028' and i == 3:
+                        import pdb; pdb.set_trace()
                     self.assertEqual(
                         reader.readline(keepends=True),
                         size*"a" + lineend,

Opened separate issue20538 for this bug.

Thank you Vajrasky. I got the same result.