Mention -b and -bb
diff --git a/Doc/howto/pyporting.rst b/Doc/howto/pyporting.rst
index 3696f4b..cb86826 100644
--- a/Doc/howto/pyporting.rst
+++ b/Doc/howto/pyporting.rst
@@ -319,6 +319,37 @@
consistently stick to that API in both Python 2 and 3.
+Bytes / unicode comparison
+**************************
+
+In Python 3, mixing bytes and unicode is forbidden in most situations; it
+will raise a :class:`TypeError` where Python 2 would have attempted an implicit
+coercion between types. However, there is one case where it doesn't and
+it can be very misleading::
+
+ >>> b"" == ""
+ False
+
+This is because comparison for equality is required by the language to always
+succeed (and return ``False`` for incompatible types). However, this also
+means that code incorrectly ported to Python 3 can display buggy behaviour
+if such comparisons are silently executed. To detect such situations,
+Python 3 has a ``-b`` flag that will display a warning::
+
+ $ python3 -b
+ >>> b"" == ""
+ __main__:1: BytesWarning: Comparison between bytes and string
+ False
+
+To turn the warning into an exception, use the ``-bb`` flag instead::
+
+ $ python3 -bb
+ >>> b"" == ""
+ Traceback (most recent call last):
+ File "<stdin>", line 1, in <module>
+ BytesWarning: Comparison between bytes and string
+
+
``__str__()``/``__unicode__()``
'''''''''''''''''''''''''''''''
In Python 2, objects can specify both a string and unicode representation of