open()ΒΆ

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 Py2.6 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 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''