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 to the appropriate future.builtins.bytes or future.builtins.str types. If you need the returned data to behave the 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''