diff --git a/newsfragments/3618.minor b/newsfragments/3618.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/crypto/util.py b/src/allmydata/crypto/util.py
index d377b6396..8b8619e47 100644
--- a/src/allmydata/crypto/util.py
+++ b/src/allmydata/crypto/util.py
@@ -30,5 +30,5 @@ def remove_prefix(s_bytes, prefix):
     if s_bytes.startswith(prefix):
         return s_bytes[len(prefix):]
     raise BadPrefixError(
-        "did not see expected '{}' prefix".format(prefix)
+        "did not see expected '{!r}' prefix".format(prefix)
     )
diff --git a/src/allmydata/hashtree.py b/src/allmydata/hashtree.py
index d8154f16e..17467459b 100644
--- a/src/allmydata/hashtree.py
+++ b/src/allmydata/hashtree.py
@@ -164,8 +164,10 @@ class CompleteBinaryTreeMixin(object):
     def dump(self):
         lines = []
         for i,depth in self.depth_first():
-            lines.append("%s%3d: %s" % ("  "*depth, i,
-                                        base32.b2a_or_none(self[i])))
+            value = base32.b2a_or_none(self[i])
+            if value is not None:
+                value = str(value, "utf-8")
+            lines.append("%s%3d: %s" % ("  "*depth, i, value))
         return "\n".join(lines) + "\n"
 
     def get_leaf_index(self, leafnum):
diff --git a/src/allmydata/immutable/checker.py b/src/allmydata/immutable/checker.py
index c1eb5480d..30abc68c6 100644
--- a/src/allmydata/immutable/checker.py
+++ b/src/allmydata/immutable/checker.py
@@ -67,12 +67,12 @@ class ValidatedExtendedURIProxy(object):
         self.crypttext_hash = None
 
     def __str__(self):
-        return "<%s %s>" % (self.__class__.__name__, self._verifycap.to_string())
+        return "<%s %r>" % (self.__class__.__name__, self._verifycap.to_string())
 
     def _check_integrity(self, data):
         h = uri_extension_hash(data)
         if h != self._verifycap.uri_extension_hash:
-            msg = ("The copy of uri_extension we received from %s was bad: wanted %s, got %s" %
+            msg = ("The copy of uri_extension we received from %s was bad: wanted %r, got %r" %
                    (self._readbucketproxy,
                     base32.b2a(self._verifycap.uri_extension_hash),
                     base32.b2a(h)))
@@ -234,7 +234,7 @@ class ValidatedReadBucketProxy(log.PrefixingLogMixin):
         UEB"""
         precondition(share_hash_tree[0] is not None, share_hash_tree)
         prefix = "%d-%s-%s" % (sharenum, bucket,
-                               base32.b2a(share_hash_tree[0][:8])[:12])
+                               str(base32.b2a(share_hash_tree[0][:8])[:12], "ascii"))
         log.PrefixingLogMixin.__init__(self,
                                        facility="tahoe.immutable.download",
                                        prefix=prefix)
@@ -427,7 +427,7 @@ class ValidatedReadBucketProxy(log.PrefixingLogMixin):
                 received from the remote peer were bad.""")
             self.log(" have candidate_share_hash: %s" % bool(candidate_share_hash))
             self.log(" block length: %d" % len(blockdata))
-            self.log(" block hash: %s" % base32.b2a_or_none(blockhash))
+            self.log(" block hash: %r" % base32.b2a_or_none(blockhash))
             if len(blockdata) < 100:
                 self.log(" block data: %r" % (blockdata,))
             else:
diff --git a/src/allmydata/immutable/downloader/fetcher.py b/src/allmydata/immutable/downloader/fetcher.py
index 6570346de..4e8b7f926 100644
--- a/src/allmydata/immutable/downloader/fetcher.py
+++ b/src/allmydata/immutable/downloader/fetcher.py
@@ -127,7 +127,7 @@ class SegmentFetcher(object):
                 # we could have sent something if we'd been allowed to pull
                 # more shares per server. Increase the limit and try again.
                 self._max_shares_per_server += 1
-                log.msg("SegmentFetcher(%s) increasing diversity limit to %d"
+                log.msg("SegmentFetcher(%r) increasing diversity limit to %d"
                         % (self._node._si_prefix, self._max_shares_per_server),
                         level=log.NOISY, umid="xY2pBA")
                 # Also ask for more shares, in the hopes of achieving better
diff --git a/src/allmydata/immutable/downloader/node.py b/src/allmydata/immutable/downloader/node.py
index c01fa22cc..10ce0e5c7 100644
--- a/src/allmydata/immutable/downloader/node.py
+++ b/src/allmydata/immutable/downloader/node.py
@@ -500,7 +500,7 @@ class DownloadNode(object):
             return (offset, segment, decodetime)
         except (BadHashError, NotEnoughHashesError):
             format = ("hash failure in ciphertext_hash_tree:"
-                      " segnum=%(segnum)d, SI=%(si)s")
+                      " segnum=%(segnum)d, SI=%(si)r")
             log.msg(format=format, segnum=segnum, si=self._si_prefix,
                     failure=Failure(),
                     level=log.WEIRD, parent=self._lp, umid="MTwNnw")
diff --git a/src/allmydata/immutable/downloader/segmentation.py b/src/allmydata/immutable/downloader/segmentation.py
index b1d42dcc4..6d7cb7676 100644
--- a/src/allmydata/immutable/downloader/segmentation.py
+++ b/src/allmydata/immutable/downloader/segmentation.py
@@ -120,7 +120,7 @@ class Segmentation(object):
             # we didn't get the first byte, so we can't use this segment
             log.msg("Segmentation handed wrong data:"
                     " want [%d-%d), given [%d-%d), for segnum=%d,"
-                    " for si=%s"
+                    " for si=%r"
                     % (self._offset, self._offset+self._size,
                        segment_start, segment_start+len(segment),
                        wanted_segnum, self._node._si_prefix),
diff --git a/src/allmydata/immutable/downloader/share.py b/src/allmydata/immutable/downloader/share.py
index f279018ba..86b99e99c 100644
--- a/src/allmydata/immutable/downloader/share.py
+++ b/src/allmydata/immutable/downloader/share.py
@@ -108,7 +108,7 @@ class Share(object):
         self.had_corruption = False # for unit tests
 
     def __repr__(self):
-        return "Share(sh%d-on-%s)" % (self._shnum, self._server.get_name())
+        return "Share(sh%d-on-%s)" % (self._shnum, str(self._server.get_name(), "utf-8"))
 
     def is_alive(self):
         # XXX: reconsider. If the share sees a single error, should it remain
diff --git a/src/allmydata/immutable/layout.py b/src/allmydata/immutable/layout.py
index 6736e4e06..3db9f096e 100644
--- a/src/allmydata/immutable/layout.py
+++ b/src/allmydata/immutable/layout.py
@@ -175,7 +175,7 @@ class WriteBucketProxy(object):
         self._offset_data = offset_data
 
     def __repr__(self):
-        return "<WriteBucketProxy for node %s>" % self._server.get_name()
+        return "<WriteBucketProxy for node %r>" % self._server.get_name()
 
     def put_header(self):
         return self._write(0, self._offset_data)
@@ -317,7 +317,7 @@ class ReadBucketProxy(object):
         return self._server.get_serverid()
 
     def __repr__(self):
-        return "<ReadBucketProxy %s to peer [%s] SI %s>" % \
+        return "<ReadBucketProxy %r to peer [%r] SI %r>" % \
                (id(self), self._server.get_name(), si_b2a(self._storage_index))
 
     def _start_if_needed(self):
diff --git a/src/allmydata/immutable/offloaded.py b/src/allmydata/immutable/offloaded.py
index 2d2c5c1f5..2c4b9db78 100644
--- a/src/allmydata/immutable/offloaded.py
+++ b/src/allmydata/immutable/offloaded.py
@@ -81,7 +81,7 @@ class CHKCheckerAndUEBFetcher(object):
     def _got_response(self, buckets, server):
         # buckets is a dict: maps shum to an rref of the server who holds it
         shnums_s = ",".join([str(shnum) for shnum in buckets])
-        self.log("got_response: [%s] has %d shares (%s)" %
+        self.log("got_response: [%r] has %d shares (%s)" %
                  (server.get_name(), len(buckets), shnums_s),
                  level=log.NOISY)
         self._found_shares.update(buckets.keys())
@@ -167,7 +167,7 @@ class CHKUploadHelper(Referenceable, upload.CHKUploader):  # type: ignore # warn
         self._upload_status.set_storage_index(storage_index)
         self._upload_status.set_status("fetching ciphertext")
         self._upload_status.set_progress(0, 1.0)
-        self._helper.log("CHKUploadHelper starting for SI %s" % self._upload_id,
+        self._helper.log("CHKUploadHelper starting for SI %r" % self._upload_id,
                          parent=log_number)
 
         self._storage_broker = storage_broker
diff --git a/src/allmydata/immutable/upload.py b/src/allmydata/immutable/upload.py
index b2c4c744d..3d794abf1 100644
--- a/src/allmydata/immutable/upload.py
+++ b/src/allmydata/immutable/upload.py
@@ -278,7 +278,7 @@ class ServerTracker(object):
         self.cancel_secret = bucket_cancel_secret
 
     def __repr__(self):
-        return ("<ServerTracker for server %s and SI %s>"
+        return ("<ServerTracker for server %r and SI %r>"
                 % (self._server.get_name(), si_b2a(self.storage_index)[:5]))
 
     def get_server(self):
@@ -338,7 +338,7 @@ class ServerTracker(object):
 
 
 def str_shareloc(shnum, bucketwriter):
-    return "%s: %s" % (shnum, bucketwriter.get_servername(),)
+    return "%s: %s" % (shnum, ensure_str(bucketwriter.get_servername()),)
 
 
 @implementer(IPeerSelector)
@@ -590,7 +590,7 @@ class Tahoe2ServerSelector(log.PrefixingLogMixin):
             d = timeout_call(self._reactor, tracker.ask_about_existing_shares(), 15)
             d.addBoth(self._handle_existing_response, tracker)
             ds.append(d)
-            self.log("asking server %s for any existing shares" %
+            self.log("asking server %r for any existing shares" %
                      (tracker.get_name(),), level=log.NOISY)
 
         for tracker in write_trackers:
@@ -605,7 +605,7 @@ class Tahoe2ServerSelector(log.PrefixingLogMixin):
             d.addErrback(timed_out, tracker)
             d.addBoth(self._handle_existing_write_response, tracker, set())
             ds.append(d)
-            self.log("asking server %s for any existing shares" %
+            self.log("asking server %r for any existing shares" %
                      (tracker.get_name(),), level=log.NOISY)
 
         trackers = set(write_trackers) | set(readonly_trackers)
@@ -749,7 +749,7 @@ class Tahoe2ServerSelector(log.PrefixingLogMixin):
             buckets = res
             if buckets:
                 self.serverids_with_shares.add(serverid)
-            self.log("response to get_buckets() from server %s: alreadygot=%s"
+            self.log("response to get_buckets() from server %r: alreadygot=%s"
                     % (tracker.get_name(), tuple(sorted(buckets))),
                     level=log.NOISY)
             for bucket in buckets:
@@ -818,7 +818,7 @@ class Tahoe2ServerSelector(log.PrefixingLogMixin):
                     self.homeless_shares.remove(shnum)
 
         if self._status:
-            self._status.set_status("Contacting Servers [%s] (first query),"
+            self._status.set_status("Contacting Servers [%r] (first query),"
                                     " %d shares left.."
                                     % (tracker.get_name(),
                                        len(self.homeless_shares)))
@@ -845,7 +845,7 @@ class Tahoe2ServerSelector(log.PrefixingLogMixin):
 
         else:
             (alreadygot, allocated) = res
-            self.log("response to allocate_buckets() from server %s: alreadygot=%s, allocated=%s"
+            self.log("response to allocate_buckets() from server %r: alreadygot=%s, allocated=%s"
                     % (tracker.get_name(),
                        tuple(sorted(alreadygot)), tuple(sorted(allocated))),
                     level=log.NOISY)
diff --git a/src/allmydata/introducer/server.py b/src/allmydata/introducer/server.py
index 339c5a0ac..dcc2fd2c0 100644
--- a/src/allmydata/introducer/server.py
+++ b/src/allmydata/introducer/server.py
@@ -300,7 +300,7 @@ class IntroducerService(service.MultiService, Referenceable):
                          level=log.UNUSUAL, umid="jfGMXQ")
 
     def remote_subscribe_v2(self, subscriber, service_name, subscriber_info):
-        self.log("introducer: subscription[%s] request at %s"
+        self.log("introducer: subscription[%r] request at %r"
                  % (service_name, subscriber), umid="U3uzLg")
         service_name = ensure_text(service_name)
         subscriber_info = dictutil.UnicodeKeyDict({
diff --git a/src/allmydata/mutable/checker.py b/src/allmydata/mutable/checker.py
index e3e5951f0..0899168c3 100644
--- a/src/allmydata/mutable/checker.py
+++ b/src/allmydata/mutable/checker.py
@@ -9,6 +9,7 @@ from __future__ import unicode_literals
 from future.utils import PY2
 if PY2:
     from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
+from six import ensure_str
 
 from allmydata.uri import from_string
 from allmydata.util import base32, log, dictutil
@@ -202,7 +203,7 @@ class MutableChecker(object):
             serverid = server.get_serverid()
             locator = (server, self._storage_index, shnum)
             corrupt_share_locators.append(locator)
-            s = "%s-sh%d" % (server.get_name(), shnum)
+            s = "%s-sh%d" % (ensure_str(server.get_name()), shnum)
             if f.check(CorruptShareError):
                 ft = f.value.reason
             else:
diff --git a/src/allmydata/mutable/common.py b/src/allmydata/mutable/common.py
index 802681ae7..87951c7b2 100644
--- a/src/allmydata/mutable/common.py
+++ b/src/allmydata/mutable/common.py
@@ -63,7 +63,7 @@ class CorruptShareError(BadShareError):
         self.shnum = shnum
         self.reason = reason
     def __str__(self):
-        return "<CorruptShareError server=%s shnum[%d]: %s" % \
+        return "<CorruptShareError server=%r shnum[%d]: %s" % \
                (self.server.get_name(), self.shnum, self.reason)
 
 class UnknownVersionError(BadShareError):
diff --git a/src/allmydata/mutable/filenode.py b/src/allmydata/mutable/filenode.py
index 39e8b76be..baedacb23 100644
--- a/src/allmydata/mutable/filenode.py
+++ b/src/allmydata/mutable/filenode.py
@@ -98,7 +98,7 @@ class MutableFileNode(object):
 
     def __repr__(self):
         if hasattr(self, '_uri'):
-            return "<%s %x %s %s>" % (self.__class__.__name__, id(self), self.is_readonly() and 'RO' or 'RW', self._uri.abbrev())
+            return "<%s %x %s %r>" % (self.__class__.__name__, id(self), self.is_readonly() and 'RO' or 'RW', self._uri.abbrev())
         else:
             return "<%s %x %s %s>" % (self.__class__.__name__, id(self), None, None)
 
diff --git a/src/allmydata/mutable/publish.py b/src/allmydata/mutable/publish.py
index 86ceb022c..a7bca6cba 100644
--- a/src/allmydata/mutable/publish.py
+++ b/src/allmydata/mutable/publish.py
@@ -915,7 +915,7 @@ class Publish(object):
     def log_goal(self, goal, message=""):
         logmsg = [message]
         for (shnum, server) in sorted([(s,p) for (p,s) in goal], key=lambda t: (id(t[0]), id(t[1]))):
-            logmsg.append("sh%d to [%s]" % (shnum, server.get_name()))
+            logmsg.append("sh%d to [%r]" % (shnum, server.get_name()))
         self.log("current goal: %s" % (", ".join(logmsg)), level=log.NOISY)
         self.log("we are planning to push new seqnum=#%d" % self._new_seqnum,
                  level=log.NOISY)
@@ -999,7 +999,7 @@ class Publish(object):
             return
 
         server = writer.server
-        lp = self.log("_got_write_answer from %s, share %d" %
+        lp = self.log("_got_write_answer from %r, share %d" %
                       (server.get_name(), writer.shnum))
 
         now = time.time()
@@ -1135,14 +1135,14 @@ class Publish(object):
                     (seqnum, root_hash, IV, segsize, datalength, k, N, prefix,
                      offsets_tuple) = expected_version
                     msg = ("somebody modified the share on us:"
-                           " shnum=%d: I thought they had #%d:R=%s," %
+                           " shnum=%d: I thought they had #%d:R=%r," %
                            (shnum,
                             seqnum, base32.b2a(root_hash)[:4]))
                     if unknown_format:
                         msg += (" but I don't know how to read share"
                                 " format %d" % version)
                     else:
-                        msg += " but testv reported #%d:R=%s" % \
+                        msg += " but testv reported #%d:R=%r" % \
                                (other_seqnum, base32.b2a(other_roothash)[:4])
                     self.log(msg, parent=lp, level=log.NOISY)
                 # if expected_version==None, then we didn't expect to see a
diff --git a/src/allmydata/mutable/retrieve.py b/src/allmydata/mutable/retrieve.py
index c6d4e2e40..32aaa72e5 100644
--- a/src/allmydata/mutable/retrieve.py
+++ b/src/allmydata/mutable/retrieve.py
@@ -574,7 +574,7 @@ class Retrieve(object):
         remote server (with no guarantee of success) that its share is
         corrupt.
         """
-        self.log("marking share %d on server %s as bad" % \
+        self.log("marking share %d on server %r as bad" % \
                  (shnum, server.get_name()))
         prefix = self.verinfo[-2]
         self.servermap.mark_bad_share(server, shnum, prefix)
diff --git a/src/allmydata/mutable/servermap.py b/src/allmydata/mutable/servermap.py
index be8033f4d..d793d1dcd 100644
--- a/src/allmydata/mutable/servermap.py
+++ b/src/allmydata/mutable/servermap.py
@@ -11,6 +11,7 @@ if PY2:
     # Doesn't import str to prevent API leakage on Python 2
     from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, max, min  # noqa: F401
 from past.builtins import unicode
+from six import ensure_str
 
 import sys, time, copy
 from zope.interface import implementer
@@ -202,8 +203,8 @@ class ServerMap(object):
             (seqnum, root_hash, IV, segsize, datalength, k, N, prefix,
              offsets_tuple) = verinfo
             print("[%s]: sh#%d seq%d-%s %d-of-%d len%d" %
-                         (server.get_name(), shnum,
-                          seqnum, base32.b2a(root_hash)[:4], k, N,
+                         (unicode(server.get_name(), "utf-8"), shnum,
+                          seqnum, unicode(base32.b2a(root_hash)[:4], "utf-8"), k, N,
                           datalength), file=out)
         if self._problems:
             print("%d PROBLEMS" % len(self._problems), file=out)
@@ -869,7 +870,7 @@ class ServermapUpdater(object):
         # versions.
         self.log(" found valid version %d-%s from %s-sh%d: %d-%d/%d/%d"
                  % (seqnum, unicode(base32.b2a(root_hash)[:4], "utf-8"),
-                    server.get_name(), shnum,
+                    ensure_str(server.get_name()), shnum,
                     k, n, segsize, datalen),
                     parent=lp)
         self._valid_versions.add(verinfo)
@@ -943,13 +944,13 @@ class ServermapUpdater(object):
         alleged_privkey_s = self._node._decrypt_privkey(enc_privkey)
         alleged_writekey = hashutil.ssk_writekey_hash(alleged_privkey_s)
         if alleged_writekey != self._node.get_writekey():
-            self.log("invalid privkey from %s shnum %d" %
+            self.log("invalid privkey from %r shnum %d" %
                      (server.get_name(), shnum),
                      parent=lp, level=log.WEIRD, umid="aJVccw")
             return
 
         # it's good
-        self.log("got valid privkey from shnum %d on serverid %s" %
+        self.log("got valid privkey from shnum %d on serverid %r" %
                  (shnum, server.get_name()),
                  parent=lp)
         privkey, _ = rsa.create_signing_keypair_from_string(alleged_privkey_s)
@@ -1211,9 +1212,9 @@ class ServermapUpdater(object):
                 break
             more_queries.append(self.extra_servers.pop(0))
 
-        self.log(format="sending %(more)d more queries: %(who)s",
+        self.log(format="sending %(more)d more queries: %(who)d",
                  more=len(more_queries),
-                 who=" ".join(["[%s]" % s.get_name() for s in more_queries]),
+                 who=" ".join(["[%r]" % s.get_name() for s in more_queries]),
                  level=log.NOISY)
 
         for server in more_queries:
diff --git a/src/allmydata/scripts/debug.py b/src/allmydata/scripts/debug.py
index e4bcf165e..b8aeee91e 100644
--- a/src/allmydata/scripts/debug.py
+++ b/src/allmydata/scripts/debug.py
@@ -6,6 +6,7 @@ except ImportError:
     pass
 
 from future.utils import bchr
+from past.builtins import unicode
 
 # do not import any allmydata modules at this level. Do that from inside
 # individual functions instead.
@@ -90,27 +91,34 @@ def dump_immutable_chk_share(f, out, options):
              "crypttext_hash", "crypttext_root_hash",
              "share_root_hash", "UEB_hash")
     display_keys = {"size": "file_size"}
+
+    def to_string(v):
+        if isinstance(v, bytes):
+            return unicode(v, "utf-8")
+        else:
+            return str(v)
+
     for k in keys1:
         if k in unpacked:
             dk = display_keys.get(k, k)
-            print("%20s: %s" % (dk, unpacked[k]), file=out)
+            print("%20s: %s" % (dk, to_string(unpacked[k])), file=out)
     print(file=out)
     for k in keys2:
         if k in unpacked:
             dk = display_keys.get(k, k)
-            print("%20s: %s" % (dk, unpacked[k]), file=out)
+            print("%20s: %s" % (dk, to_string(unpacked[k])), file=out)
     print(file=out)
     for k in keys3:
         if k in unpacked:
             dk = display_keys.get(k, k)
-            print("%20s: %s" % (dk, unpacked[k]), file=out)
+            print("%20s: %s" % (dk, to_string(unpacked[k])), file=out)
 
     leftover = set(unpacked.keys()) - set(keys1 + keys2 + keys3)
     if leftover:
         print(file=out)
         print("LEFTOVER:", file=out)
         for k in sorted(leftover):
-            print("%20s: %s" % (k, unpacked[k]), file=out)
+            print("%20s: %s" % (k, to_string(unpacked[k])), file=out)
 
     # the storage index isn't stored in the share itself, so we depend upon
     # knowing the parent directory name to get it
@@ -197,7 +205,7 @@ def dump_mutable_share(options):
     print(file=out)
     print("Mutable slot found:", file=out)
     print(" share_type: %s" % share_type, file=out)
-    print(" write_enabler: %s" % base32.b2a(WE), file=out)
+    print(" write_enabler: %s" % unicode(base32.b2a(WE), "utf-8"), file=out)
     print(" WE for nodeid: %s" % idlib.nodeid_b2a(nodeid), file=out)
     print(" num_extra_leases: %d" % num_extra_leases, file=out)
     print(" container_size: %d" % container_size, file=out)
@@ -209,8 +217,8 @@ def dump_mutable_share(options):
             print("  ownerid: %d" % lease.owner_num, file=out)
             when = format_expiration_time(lease.expiration_time)
             print("  expires in %s" % when, file=out)
-            print("  renew_secret: %s" % base32.b2a(lease.renew_secret), file=out)
-            print("  cancel_secret: %s" % base32.b2a(lease.cancel_secret), file=out)
+            print("  renew_secret: %s" % unicode(base32.b2a(lease.renew_secret), "utf-8"), file=out)
+            print("  cancel_secret: %s" % unicode(base32.b2a(lease.cancel_secret), "utf-8"), file=out)
             print("  secrets are for nodeid: %s" % idlib.nodeid_b2a(lease.nodeid), file=out)
     else:
         print("No leases.", file=out)
@@ -258,8 +266,8 @@ def dump_SDMF_share(m, length, options):
 
     print(" SDMF contents:", file=out)
     print("  seqnum: %d" % seqnum, file=out)
-    print("  root_hash: %s" % base32.b2a(root_hash), file=out)
-    print("  IV: %s" % base32.b2a(IV), file=out)
+    print("  root_hash: %s" % unicode(base32.b2a(root_hash), "utf-8"), file=out)
+    print("  IV: %s" % unicode(base32.b2a(IV), "utf-8"), file=out)
     print("  required_shares: %d" % k, file=out)
     print("  total_shares: %d" % N, file=out)
     print("  segsize: %d" % segsize, file=out)
@@ -352,7 +360,7 @@ def dump_MDMF_share(m, length, options):
 
     print(" MDMF contents:", file=out)
     print("  seqnum: %d" % seqnum, file=out)
-    print("  root_hash: %s" % base32.b2a(root_hash), file=out)
+    print("  root_hash: %s" % unicode(base32.b2a(root_hash), "utf-8"), file=out)
     #print("  IV: %s" % base32.b2a(IV), file=out)
     print("  required_shares: %d" % k, file=out)
     print("  total_shares: %d" % N, file=out)
@@ -745,7 +753,7 @@ def describe_share(abs_sharefile, si_s, shnum_s, now, out):
 
             print("SDMF %s %d/%d %d #%d:%s %d %s" % \
                   (si_s, k, N, datalen,
-                   seqnum, base32.b2a(root_hash),
+                   seqnum, unicode(base32.b2a(root_hash), "utf-8"),
                    expiration, quote_output(abs_sharefile)), file=out)
         elif share_type == "MDMF":
             from allmydata.mutable.layout import MDMFSlotReadProxy
@@ -774,7 +782,7 @@ def describe_share(abs_sharefile, si_s, shnum_s, now, out):
              offsets) = verinfo
             print("MDMF %s %d/%d %d #%d:%s %d %s" % \
                   (si_s, k, N, datalen,
-                   seqnum, base32.b2a(root_hash),
+                   seqnum, unicode(base32.b2a(root_hash), "utf-8"),
                    expiration, quote_output(abs_sharefile)), file=out)
         else:
             print("UNKNOWN mutable %s" % quote_output(abs_sharefile), file=out)
@@ -808,8 +816,8 @@ def describe_share(abs_sharefile, si_s, shnum_s, now, out):
         ueb_hash = unpacked["UEB_hash"]
 
         print("CHK %s %d/%d %d %s %d %s" % (si_s, k, N, filesize,
-                                                   ueb_hash, expiration,
-                                                   quote_output(abs_sharefile)), file=out)
+                                            unicode(ueb_hash, "utf-8"), expiration,
+                                            quote_output(abs_sharefile)), file=out)
 
     else:
         print("UNKNOWN really-unknown %s" % quote_output(abs_sharefile), file=out)
diff --git a/src/allmydata/storage_client.py b/src/allmydata/storage_client.py
index eb1572dcb..75c554562 100644
--- a/src/allmydata/storage_client.py
+++ b/src/allmydata/storage_client.py
@@ -38,7 +38,6 @@ from future.utils import PY2
 if PY2:
     from future.builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
-
 import re, time, hashlib
 
 # On Python 2 this will be the backport.
@@ -820,7 +819,7 @@ class NativeStorageServer(service.MultiService):
         return self
 
     def __repr__(self):
-        return "<NativeStorageServer for %s>" % self.get_name()
+        return "<NativeStorageServer for %r>" % self.get_name()
     def get_serverid(self):
         return self._server_id
     def get_version(self):
@@ -844,10 +843,10 @@ class NativeStorageServer(service.MultiService):
         version = self.get_version()
         if version is None:
             return None
-        protocol_v1_version = version.get('http://allmydata.org/tahoe/protocols/storage/v1', UnicodeKeyDict())
-        available_space = protocol_v1_version.get('available-space')
+        protocol_v1_version = version.get(b'http://allmydata.org/tahoe/protocols/storage/v1', BytesKeyDict())
+        available_space = protocol_v1_version.get(b'available-space')
         if available_space is None:
-            available_space = protocol_v1_version.get('maximum-immutable-share-size', None)
+            available_space = protocol_v1_version.get(b'maximum-immutable-share-size', None)
         return available_space
 
     def start_connecting(self, trigger_cb):
diff --git a/src/allmydata/test/__init__.py b/src/allmydata/test/__init__.py
index 19c046eca..e3ac48290 100644
--- a/src/allmydata/test/__init__.py
+++ b/src/allmydata/test/__init__.py
@@ -14,13 +14,23 @@ Rather than defining interesting APIs for other code to use, this just causes
 some side-effects which make things better when the test suite runs.
 """
 
+from future.utils import PY3
+
+import warnings
 from traceback import extract_stack, format_list
+
 from foolscap.pb import Listener
 from twisted.python.log import err
 from twisted.application import service
 
-
 from foolscap.logging.incident import IncidentQualifier
+
+if PY3:
+    # Error on BytesWarnings, to catch things like str(b""), but only for
+    # allmydata code.
+    warnings.filterwarnings("error", category=BytesWarning, module="allmydata.*")
+
+
 class NonQualifier(IncidentQualifier, object):
     def check_event(self, ev):
         return False
diff --git a/src/allmydata/test/mutable/test_update.py b/src/allmydata/test/mutable/test_update.py
index 65af06486..da5d53e4c 100644
--- a/src/allmydata/test/mutable/test_update.py
+++ b/src/allmydata/test/mutable/test_update.py
@@ -114,9 +114,9 @@ class Update(GridTestMixin, unittest.TestCase, testutil.ShouldFailMixin):
         # with problems and display them separately
         gotmods = [mo.span() for mo in re.finditer(b'([A-Z]+)', got)]
         expmods = [mo.span() for mo in re.finditer(b'([A-Z]+)', expected)]
-        gotspans = ["%d:%d=%s" % (start,end,got[start:end])
+        gotspans = ["%d:%d=%r" % (start,end,got[start:end])
                     for (start,end) in gotmods]
-        expspans = ["%d:%d=%s" % (start,end,expected[start:end])
+        expspans = ["%d:%d=%r" % (start,end,expected[start:end])
                     for (start,end) in expmods]
         #print("expecting: %s" % expspans)
 
diff --git a/src/allmydata/test/no_network.py b/src/allmydata/test/no_network.py
index cbea0dfcd..2f75f9274 100644
--- a/src/allmydata/test/no_network.py
+++ b/src/allmydata/test/no_network.py
@@ -200,7 +200,8 @@ class NoNetworkServer(object):
         return self.serverid
 
     def get_name(self):
-        return idlib.shortnodeid_b2a(self.serverid)
+        # Other implementations return bytes.
+        return idlib.shortnodeid_b2a(self.serverid).encode("utf-8")
     def get_longname(self):
         return idlib.nodeid_b2a(self.serverid)
     def get_nickname(self):
diff --git a/src/allmydata/test/test_storage_client.py b/src/allmydata/test/test_storage_client.py
index 8500d6bff..1a84f35ec 100644
--- a/src/allmydata/test/test_storage_client.py
+++ b/src/allmydata/test/test_storage_client.py
@@ -118,17 +118,17 @@ class NativeStorageServerWithVersion(NativeStorageServer):  # type: ignore  # ta
 class TestNativeStorageServer(unittest.TestCase):
     def test_get_available_space_new(self):
         nss = NativeStorageServerWithVersion(
-            { "http://allmydata.org/tahoe/protocols/storage/v1":
-                { "maximum-immutable-share-size": 111,
-                  "available-space": 222,
+            { b"http://allmydata.org/tahoe/protocols/storage/v1":
+                { b"maximum-immutable-share-size": 111,
+                  b"available-space": 222,
                 }
             })
         self.failUnlessEqual(nss.get_available_space(), 222)
 
     def test_get_available_space_old(self):
         nss = NativeStorageServerWithVersion(
-            { "http://allmydata.org/tahoe/protocols/storage/v1":
-                { "maximum-immutable-share-size": 111,
+            { b"http://allmydata.org/tahoe/protocols/storage/v1":
+                { b"maximum-immutable-share-size": 111,
                 }
             })
         self.failUnlessEqual(nss.get_available_space(), 111)
diff --git a/src/allmydata/test/test_system.py b/src/allmydata/test/test_system.py
index 8214c312c..ce575ce7a 100644
--- a/src/allmydata/test/test_system.py
+++ b/src/allmydata/test/test_system.py
@@ -1072,7 +1072,7 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
         d.addCallback(_do_upload)
         def _upload_done(results):
             theuri = results.get_uri()
-            log.msg("upload finished: uri is %s" % (theuri,))
+            log.msg("upload finished: uri is %r" % (theuri,))
             self.uri = theuri
             assert isinstance(self.uri, bytes), self.uri
             self.cap = uri.from_string(self.uri)
diff --git a/src/allmydata/uri.py b/src/allmydata/uri.py
index a659dac38..70742b7b2 100644
--- a/src/allmydata/uri.py
+++ b/src/allmydata/uri.py
@@ -99,7 +99,7 @@ class CHKFileURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)),
                    int(mo.group(3)), int(mo.group(4)), int(mo.group(5)))
 
@@ -290,7 +290,7 @@ class ReadonlySSKFileURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
@@ -300,7 +300,7 @@ class ReadonlySSKFileURI(_BaseURI):
                                       base32.b2a(self.fingerprint))
 
     def __repr__(self):
-        return "<%s %s>" % (self.__class__.__name__, self.abbrev())
+        return "<%s %r>" % (self.__class__.__name__, self.abbrev())
 
     def abbrev(self):
         return base32.b2a(self.readkey[:5])
@@ -336,7 +336,7 @@ class SSKVerifierURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
@@ -375,7 +375,7 @@ class WriteableMDMFFileURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
@@ -386,7 +386,7 @@ class WriteableMDMFFileURI(_BaseURI):
         return ret
 
     def __repr__(self):
-        return "<%s %s>" % (self.__class__.__name__, self.abbrev())
+        return "<%s %r>" % (self.__class__.__name__, self.abbrev())
 
     def abbrev(self):
         return base32.b2a(self.writekey[:5])
@@ -423,7 +423,7 @@ class ReadonlyMDMFFileURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
 
         return cls(base32.a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
@@ -435,7 +435,7 @@ class ReadonlyMDMFFileURI(_BaseURI):
         return ret
 
     def __repr__(self):
-        return "<%s %s>" % (self.__class__.__name__, self.abbrev())
+        return "<%s %r>" % (self.__class__.__name__, self.abbrev())
 
     def abbrev(self):
         return base32.b2a(self.readkey[:5])
@@ -471,7 +471,7 @@ class MDMFVerifierURI(_BaseURI):
     def init_from_string(cls, uri):
         mo = cls.STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         return cls(si_a2b(mo.group(1)), base32.a2b(mo.group(2)))
 
     def to_string(self):
@@ -500,13 +500,13 @@ class _DirectoryBaseURI(_BaseURI):
         self._filenode_uri = filenode_uri
 
     def __repr__(self):
-        return "<%s %s>" % (self.__class__.__name__, self.abbrev())
+        return "<%s %r>" % (self.__class__.__name__, self.abbrev())
 
     @classmethod
     def init_from_string(cls, uri):
         mo = cls.BASE_STRING_RE.search(uri)
         if not mo:
-            raise BadURIError("'%s' doesn't look like a %s cap" % (uri, cls))
+            raise BadURIError("%r doesn't look like a %s cap" % (uri, cls))
         bits = uri[mo.end():]
         fn = cls.INNER_URI_CLASS.init_from_string(
             cls.INNER_URI_CLASS.BASE_STRING+bits)
diff --git a/src/allmydata/web/status.py b/src/allmydata/web/status.py
index 82dc411d4..6a8f689b3 100644
--- a/src/allmydata/web/status.py
+++ b/src/allmydata/web/status.py
@@ -76,7 +76,7 @@ class UploadResultsRendererMixin(Element):
             ul = tags.ul()
             for server, shnums in sorted(servermap.items(), key=id):
                 shares_s = ",".join(["#%d" % shnum for shnum in shnums])
-                ul(tags.li("[%s] got share%s: %s" % (server.get_name(),
+                ul(tags.li("[%s] got share%s: %s" % (unicode(server.get_name(), "utf-8"),
                                                      plural(shnums), shares_s)))
             return ul
         d.addCallback(_render)
@@ -230,7 +230,9 @@ class UploadStatusElement(UploadResultsRendererMixin):
         si_s = base32.b2a_or_none(self._upload_status.get_storage_index())
         if si_s is None:
             si_s = "(None)"
-        return tag(str(si_s))
+        else:
+            si_s = unicode(si_s, "utf-8")
+        return tag(si_s)
 
     @renderer
     def helper(self, req, tag):
@@ -920,7 +922,7 @@ class RetrieveStatusElement(Element):
         for server in sorted(per_server.keys(), key=lambda s: s.get_name()):
             times_s = ", ".join([abbreviate_time(t)
                                  for t in per_server[server]])
-            l(tags.li("[%s]: %s" % (server.get_name(), times_s)))
+            l(tags.li("[%s]: %s" % (unicode(server.get_name(), "utf-8"), times_s)))
         return tags.li("Per-Server Fetch Response Times: ", l)
 
 
@@ -958,7 +960,9 @@ class PublishStatusElement(Element):
         si_s = base32.b2a_or_none(self._publish_status.get_storage_index())
         if si_s is None:
             si_s = "(None)"
-        return tag(str(si_s))
+        else:
+            si_s = unicode("utf-8")
+        return tag(si_s)
 
     @renderer
     def helper(self, req, tag):
@@ -996,7 +1000,7 @@ class PublishStatusElement(Element):
         sharemap = servermap.make_sharemap()
         for shnum in sorted(sharemap.keys()):
             l(tags.li("%d -> Placed on " % shnum,
-                      ", ".join(["[%s]" % server.get_name()
+                      ", ".join(["[%s]" % unicode(server.get_name(), "utf-8")
                                  for server in sharemap[shnum]])))
         return tag("Sharemap:", l)
 
@@ -1078,7 +1082,7 @@ class PublishStatusElement(Element):
         for server in sorted(per_server.keys(), key=lambda s: s.get_name()):
             times_s = ", ".join([abbreviate_time(t)
                                  for t in per_server[server]])
-            l(tags.li("[%s]: %s" % (server.get_name(), times_s)))
+            l(tags.li("[%s]: %s" % (unicode(server.get_name(), "utf-8"), times_s)))
         return tags.li("Per-Server Response Times: ", l)
 
 
@@ -1204,7 +1208,7 @@ class MapupdateStatusElement(Element):
                 else:
                     times.append("privkey(" + abbreviate_time(t) + ")")
             times_s = ", ".join(times)
-            l(tags.li("[%s]: %s" % (server.get_name(), times_s)))
+            l(tags.li("[%s]: %s" % (unicode(server.get_name(), "utf-8"), times_s)))
         return tags.li("Per-Server Response Times: ", l)
 
 
diff --git a/tox.ini b/tox.ini
index 77f5faede..bc18ed519 100644
--- a/tox.ini
+++ b/tox.ini
@@ -44,7 +44,7 @@ deps =
 # more useful results.
 usedevelop = False
 # We use extras=test to get things like "mock" that are required for our unit
-# tests.
+# tests.8
 extras = test
 
 setenv =
@@ -62,20 +62,19 @@ commands =
 
          tahoe --version
 
-         !coverage: trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors} {posargs:{env:TEST_SUITE}}
+         # Run tests with -b to catch bugs like `"%s" % (some_bytes,)`. -b makes
+         # Python emit BytesWarnings, and warnings configuration in
+         # src/allmydata/tests/__init__.py turns allmydata's BytesWarnings into
+         # exceptions.
+         !coverage: python -b -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors} {posargs:{env:TEST_SUITE}}
 
          # measuring coverage is somewhat slower than not measuring coverage
          # so only do it on request.
-         coverage: coverage run -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:{env:TEST_SUITE}}
+         coverage: python -b -m coverage run -m twisted.trial {env:TAHOE_LAFS_TRIAL_ARGS:--rterrors --reporter=timing} {posargs:{env:TEST_SUITE}}
          coverage: coverage combine
          coverage: coverage xml
          coverage: coverage report
 
-         # Also run tests with -bb to catch bugs like `"%s" % (some_bytes,)`.
-         # Eventually everything should run with this, but so far only fixed
-         # some of the Python3-ported modules.
-         python -bb -m twisted.trial --rterrors allmydata.test.web
-
 [testenv:integration]
 setenv =
          COVERAGE_PROCESS_START=.coveragerc