The Python 3 builtin 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 read return Py3 bytes objects.
On Py2 with future installed, the builtins module provides an open function 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 io module on 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 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 builtins import open, bytes
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''