Currently (135c6a56e55d2f4f8718b3b9f03ce3c692b15f0f) the following test fails:

./python -m test.regrtest -v test_tools.test_unparse -u cpu

(Note that -u cpu is needed to force test_unparse.DirectoryTestCase to check all files under Lib/ and Lib/test/.)

An example error message:

======================================================================
FAIL: test_files (test.test_tools.test_unparse.DirectoryTestCase) (filename='/home/yen/Projects/cpython/Lib/test/test_typing.py'
)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/yen/Projects/cpython/Lib/test/test_tools/test_unparse.py", line 309, in test_files
    self.check_roundtrip(source)
  File "/home/yen/Projects/cpython/Lib/test/test_tools/test_unparse.py", line 132, in check_roundtrip
    self.assertASTEqual(ast1, ast2)
  File "/home/yen/Projects/cpython/Lib/test/test_tools/test_unparse.py", line 124, in assertASTEqual
    self.assertEqual(ast.dump(ast1), ast.dump(ast2))
AssertionError: 'Modu[88178 chars]kind=\'u\')], ctx=Load())), ctx=Load()))], dec[421987 chars]=[])' != 'Modu[88178 chars]kind=No
ne)], ctx=Load())), ctx=Load()))], deco[421986 chars]=[])'

----------------------------------------------------------------------

Apparently that's because Tools/parser/unparse.py does not handle strings like u"bar" correctly. I managed to "fix" it with the following patch:

diff --git a/Tools/parser/unparse.py b/Tools/parser/unparse.py
index 385902ef4b..a25b5e49df 100644
--- a/Tools/parser/unparse.py
+++ b/Tools/parser/unparse.py
@@ -399,6 +399,8 @@ class Unparser:
         elif value is ...:
             self.write("...")
         else:
+            if t.kind == 'u':
+                self.write("u")
             self._write_constant(t.value)

     def _List(self, t):

Not sure if this is the correct approach, though.

On the other hand, if this causes troubles, feel free to just replace u"bar" with "bar" in that test. It is not really important.

Please submit a PR.

After reading Python/ast.c, I think my patch is correct, so I submitted it to GitHub as https://github.com/python/cpython/pull/13583 :)

New changeset aaf47caf35984e614d93bd8bea5227df55e0e3e6 by Miss Islington (bot) (Chih-Hsuan Yen) in branch 'master':
bpo-37053: handle strings like u"bar" correctly in Tools/parser/unparse.py (GH-13583)
https://github.com/python/cpython/commit/aaf47caf35984e614d93bd8bea5227df55e0e3e6

Confirmed that this fixes the broken buildbots:

./python -m test test_tools -vvvv -uall -m test_files
== CPython 3.8.0a4+ (heads/master:91f4380ced, May 26 2019, 17:30:21) [GCC 8.3.0]
== Linux-5.0.15-1-MANJARO-x86_64-with-glibc2.29 little-endian
== cwd: /home/pablogsal/github/cpython/build/test_python_25649
== CPU count: 12
== encodings: locale=UTF-8, FS=utf-8
Run tests sequentially
0:00:00 load avg: 0.50 [1/1] test_tools
test_files (test.test_tools.test_unparse.DirectoryTestCase) ... Testing /home/pablogsal/github/cpython/Lib/typing.py
Testing /home/pablogsal/github/cpython/Lib/test/test_typing.py
ok

Ran 1 test in 0.586s

OK

== Tests result: SUCCESS ==

1 test OK.

Total duration: 671 ms
Tests result: SUCCESS

The manual check is needed because the CI in GitHub does not run test_tools over all files so it does not take forever.