Handling bytes consistently and correctly has traditionally been one of the most difficult tasks in writing a Py2/3 compatible codebase. This is because the Python 2 bytes object is simply an alias for Python 2’s str, rather than a true implementation of the Python 3 bytes object, which is substantially different.

future contains a backport of the bytes object from Python 3 which passes most of the Python 3 tests for bytes. (See tests/test_future/ in the source tree.) You can use it as follows:

>>> from builtins import bytes
>>> b = bytes(b'ABCD')

On Py3, this is simply the builtin bytes object. On Py2, this object is a subclass of Python 2’s str that enforces the same strict separation of unicode strings and byte strings as Python 3’s bytes object:

>>> b + u'EFGH'      # TypeError
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument can't be unicode string

>>> bytes(b',').join([u'Fred', u'Bill'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: sequence item 0: expected bytes, found unicode string

>>> b == u'ABCD'

>>> b < u'abc'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: bytes() and <type 'unicode'>

In most other ways, these bytes objects have identical behaviours to Python 3’s bytes:

b = bytes(b'ABCD')
assert list(b) == [65, 66, 67, 68]
assert repr(b) == "b'ABCD'"
assert b.split(b'B') == [b'A', b'CD']

Currently the easiest way to ensure identical behaviour of byte-strings in a Py2/3 codebase is to wrap all byte-string literals b'...' in a bytes() call as follows:

from builtins import bytes

# ...

b = bytes(b'This is my bytestring')

# ...

This is not perfect, but it is superior to manually debugging and fixing code incompatibilities caused by the many differences between Py3 bytes and Py2 strings.

The bytes type from builtins also provides support for the surrogateescape error handler on Python 2.x. Here is an example that works identically on Python 2.x and 3.x:

>>> from builtins import bytes
>>> b = bytes(b'\xff')
>>> b.decode('utf-8', 'surrogateescape')

This feature is in alpha. Please leave feedback here about whether this works for you.