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 it returns a Py3 :class:`bytes` object.

future.builtins provides a compatible open function on Py2, which uses the open backport available in the standard library :mod:`io` module on Py2.6+.

When porting code from Python 2, be aware of the different return types from methods such as :func:`read()` from the file object that open returns.

Note that the output of :func:`read()` etc. is not automatically cast to the appropriate future.builtins.bytes or future.builtins.str type. If you need the stricter type-checking of Py3 on Py2 as well, 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''