The Python 3 builtin :func:`open` function for opening files returns file contents as (unicode) strings unless the binary (b) flag is passed, as in:

open(filename, 'rb')

in which case its methods like :func:`read` return Py3 :class:`bytes` objects.

future.builtins provides an open function on Py2 that is mostly compatible with that on Python 3 (e.g. it offers keyword arguments like encoding). This maps to the open backport available in the standard library :mod:`io` module on Py2.6 and Py2.7.

One difference to be aware of between the Python 3 open and future.builtins.open on Python 2 is that the return types of methods such as :func:`read()` from the file object that open returns are not automatically cast from native bytes or unicode strings on Python 2 to the corresponding future.builtins.bytes or future.builtins.str types. If you need the returned data to behave the exactly same way on Py2 as on Py3, you can cast it explicitly as follows:

from __future__ import unicode_literals
from future.builtins import *

data = open('image.png', 'rb').read()
# On Py2, data is a standard 8-bit str with loose Unicode coercion.
# data + u'' would likely raise a UnicodeDecodeError

data = bytes(data)
# Now it behaves like a Py3 bytes object...

assert data[:4] == b'\x89PNG'
assert data[4] == 13     # integer
# Raises TypeError:
# data + u''