diff --git a/mercurial/pycompat.py b/mercurial/pycompat.py
--- a/mercurial/pycompat.py
+++ b/mercurial/pycompat.py
@@ -506,6 +506,17 @@
     return tempfile.mkstemp(suffix, prefix, dir)
 
 
+# TemporaryFile does not support an "encoding=" argument on python2.
+# This wrapper file are always open in byte mode.
+def unnamedtempfile(mode=None, *args, **kwargs):
+    if mode is None:
+        mode = b'w+b'
+    else:
+        mode = sysstr(mode)
+    assert 'b' in mode
+    return tempfile.TemporaryFile(mode, *args, **kwargs)
+
+
 # NamedTemporaryFile does not support an "encoding=" argument on python2.
 # This wrapper file are always open in byte mode.
 def namedtempfile(