Special logic for roundtripping Unicode to Unicode is only necessary on Python 2.

src/allmydata/test/test_util.py

@@ -480,7 +480,12 @@ class EqButNotIs(object):
 
 class YAML(unittest.TestCase):
     def test_convert(self):
-        data = yaml.safe_dump(["str", u"unicode", u"\u1234nicode"])
+        """
+        Unicode and (ASCII) native strings get roundtripped to Unicode strings.
+        """
+        data = yaml.safe_dump(
+            [six.ensure_str("str"), u"unicode", u"\u1234nicode"]
+        )
         back = yamlutil.safe_load(data)
         self.assertIsInstance(back[0], str)
         self.assertIsInstance(back[1], str)

src/allmydata/util/yamlutil.py

@@ -1,11 +1,28 @@
+from future.utils import PY2
+
 import yaml
 
-# Announcements contain unicode, because they come from JSON. We tell PyYAML
-# to give us unicode instead of str/bytes.
-def construct_unicode(loader, node):
-    return node.value
-yaml.SafeLoader.add_constructor("tag:yaml.org,2002:str",
-                                construct_unicode)
+if PY2:
+    # On Python 2 the way pyyaml deals with Unicode strings is inconsistent.
+    #
+    # >>> yaml.safe_load(yaml.safe_dump(u"hello"))
+    # 'hello'
+    # >>> yaml.safe_load(yaml.safe_dump(u"hello\u1234"))
+    # u'hello\u1234'
+    #
+    # In other words, Unicode strings get roundtripped to byte strings, but
+    # only sometimes.
+    #
+    # In order to ensure unicode stays unicode, we add a configuration saying
+    # that the YAML String Language-Independent Type ("a sequence of zero or
+    # more Unicode characters") should be the underlying Unicode string object,
+    # rather than converting to bytes when possible.
+    #
+    # Reference: https://yaml.org/type/str.html
+    def construct_unicode(loader, node):
+        return node.value
+    yaml.SafeLoader.add_constructor("tag:yaml.org,2002:str",
+                                    construct_unicode)
 
 def safe_load(f):
     return yaml.safe_load(f)