diff --git a/contrib/hgfixes/fix_bytes.py b/contrib/hgfixes/fix_bytes.py
--- a/contrib/hgfixes/fix_bytes.py
+++ b/contrib/hgfixes/fix_bytes.py
@@ -13,6 +13,7 @@ from lib2to3.pygram import python_symbol
 # blacklisting some modules inside the fixers. So, this is what I came with.
 
 blacklist = ['mercurial/demandimport.py',
+             'mercurial/py3kcompat.py', # valid python 3 already
              'mercurial/i18n.py',
             ]
 
diff --git a/mercurial/py3kcompat.py b/mercurial/py3kcompat.py
new file mode 100644
--- /dev/null
+++ b/mercurial/py3kcompat.py
@@ -0,0 +1,65 @@
+# py3kcompat.py - compatibility definitions for running hg in py3k
+#
+# Copyright 2010 Renato Cunha <renatoc@gmail.com>
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+import os, builtins
+
+from numbers import Number
+
+def bytesformatter(format, args):
+    '''Custom implementation of a formatter for bytestrings.
+
+    This function currently relias on the string formatter to do the
+    formatting and always returns bytes objects.
+
+    >>> bytesformatter(20, 10)
+    0
+    >>> bytesformatter('unicode %s, %s!', ('string', 'foo'))
+    b'unicode string, foo!'
+    >>> bytesformatter(b'test %s', 'me')
+    b'test me'
+    >>> bytesformatter('test %s', 'me')
+    b'test me'
+    >>> bytesformatter(b'test %s', b'me')
+    b'test me'
+    >>> bytesformatter('test %s', b'me')
+    b'test me'
+    >>> bytesformatter('test %d: %s', (1, b'result'))
+    b'test 1: result'
+    '''
+    # The current implementation just converts from bytes to unicode, do
+    # what's needed and then convert the results back to bytes.
+    # Another alternative is to use the Python C API implementation.
+    if isinstance(format, Number):
+        # If the fixer erroneously passes a number remainder operation to
+        # bytesformatter, we just return the correct operation
+        return format % args
+    if isinstance(format, bytes):
+        format = format.decode('utf-8', 'surrogateescape')
+    if isinstance(args, bytes):
+        args = args.decode('utf-8', 'surrogateescape')
+    if isinstance(args, tuple):
+        newargs = []
+        for arg in args:
+            if isinstance(arg, bytes):
+                arg = arg.decode('utf-8', 'surrogateescape')
+            newargs.append(arg)
+        args = tuple(newargs)
+    ret = format % args
+    return ret.encode('utf-8', 'surrogateescape')
+builtins.bytesformatter = bytesformatter
+
+# Create bytes equivalents for os.environ values
+for key in list(os.environ.keys()):
+    # UTF-8 is fine for us
+    bkey = key.encode('utf-8', 'surrogateescape')
+    bvalue = os.environ[key].encode('utf-8', 'surrogateescape')
+    os.environ[bkey] = bvalue
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
+