diff --git a/mercurial/bundlerepo.py b/mercurial/bundlerepo.py
--- a/mercurial/bundlerepo.py
+++ b/mercurial/bundlerepo.py
@@ -272,7 +272,7 @@ class bundlerepository(localrepo.localre
                                             suffix=".hg10un")
             self.tempfile = temp
 
-            with os.fdopen(fdtemp, 'wb') as fptemp:
+            with os.fdopen(fdtemp, pycompat.sysstr('wb')) as fptemp:
                 fptemp.write(header)
                 while True:
                     chunk = read(2**18)
diff --git a/mercurial/chgserver.py b/mercurial/chgserver.py
--- a/mercurial/chgserver.py
+++ b/mercurial/chgserver.py
@@ -287,9 +287,9 @@ class channeledsystem(object):
 
 _iochannels = [
     # server.ch, ui.fp, mode
-    ('cin', 'fin', 'rb'),
-    ('cout', 'fout', 'wb'),
-    ('cerr', 'ferr', 'wb'),
+    ('cin', 'fin', pycompat.sysstr('rb')),
+    ('cout', 'fout', pycompat.sysstr('wb')),
+    ('cerr', 'ferr', pycompat.sysstr('wb')),
 ]
 
 class chgcmdserver(commandserver.server):
diff --git a/mercurial/commandserver.py b/mercurial/commandserver.py
--- a/mercurial/commandserver.py
+++ b/mercurial/commandserver.py
@@ -304,8 +304,8 @@ def _protectio(ui):
     ui.flush()
     newfiles = []
     nullfd = os.open(os.devnull, os.O_RDWR)
-    for f, sysf, mode in [(ui.fin, util.stdin, 'rb'),
-                          (ui.fout, util.stdout, 'wb')]:
+    for f, sysf, mode in [(ui.fin, util.stdin, pycompat.sysstr('rb')),
+                          (ui.fout, util.stdout, pycompat.sysstr('wb'))]:
         if f is sysf:
             newfd = os.dup(f.fileno())
             os.dup2(nullfd, f.fileno())
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -585,7 +585,7 @@ def _filemerge(premerge, repo, mynode, o
         pre = "%s~%s." % (os.path.basename(fullbase), prefix)
         (fd, name) = tempfile.mkstemp(prefix=pre, suffix=ext)
         data = repo.wwritedata(ctx.path(), ctx.data())
-        f = os.fdopen(fd, "wb")
+        f = os.fdopen(fd, pycompat.sysstr("wb"))
         f.write(data)
         f.close()
         return name
diff --git a/mercurial/httppeer.py b/mercurial/httppeer.py
--- a/mercurial/httppeer.py
+++ b/mercurial/httppeer.py
@@ -20,6 +20,7 @@ from . import (
     bundle2,
     error,
     httpconnection,
+    pycompat,
     statichttprepo,
     url,
     util,
@@ -327,7 +328,7 @@ class httppeer(wireproto.wirepeer):
         try:
             # dump bundle to disk
             fd, filename = tempfile.mkstemp(prefix="hg-bundle-", suffix=".hg")
-            fh = os.fdopen(fd, "wb")
+            fh = os.fdopen(fd, pycompat.sysstr("wb"))
             d = fp.read(4096)
             while d:
                 fh.write(d)
diff --git a/mercurial/patch.py b/mercurial/patch.py
--- a/mercurial/patch.py
+++ b/mercurial/patch.py
@@ -34,6 +34,7 @@ from . import (
     mail,
     mdiff,
     pathutil,
+    pycompat,
     scmutil,
     similar,
     util,
@@ -209,7 +210,7 @@ def extract(ui, fileobj):
 
     data = {}
     fd, tmpname = tempfile.mkstemp(prefix='hg-patch-')
-    tmpfp = os.fdopen(fd, 'w')
+    tmpfp = os.fdopen(fd, pycompat.sysstr('w'))
     try:
         msg = email.Parser.Parser().parse(fileobj)
 
@@ -1055,7 +1056,7 @@ the hunk is left unchanged.
                 ncpatchfp = None
                 try:
                     # Write the initial patch
-                    f = os.fdopen(patchfd, "w")
+                    f = os.fdopen(patchfd, pycompat.sysstr("w"))
                     chunk.header.write(f)
                     chunk.write(f)
                     f.write('\n'.join(['# ' + i for i in phelp.splitlines()]))
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -26,6 +26,7 @@ from . import (
     exchange,
     peer,
     pushkey as pushkeymod,
+    pycompat,
     streamclone,
     util,
 )
@@ -961,7 +962,7 @@ def unbundle(repo, proto, heads):
 
         # write bundle data to temporary file because it can be big
         fd, tempname = tempfile.mkstemp(prefix='hg-unbundle-')
-        fp = os.fdopen(fd, 'wb+')
+        fp = os.fdopen(fd, pycompat.sysstr('wb+'))
         r = 0
         try:
             proto.getfile(fp)
diff --git a/mercurial/worker.py b/mercurial/worker.py
--- a/mercurial/worker.py
+++ b/mercurial/worker.py
@@ -157,7 +157,7 @@ def _posixworker(ui, func, staticargs, a
                 os._exit(0)
         pids.add(pid)
     os.close(wfd)
-    fp = os.fdopen(rfd, 'rb', 0)
+    fp = os.fdopen(rfd, pycompat.sysstr('rb'), 0)
     def cleanup():
         signal.signal(signal.SIGINT, oldhandler)
         waitforworkers()