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+.. _open-function:
+
+open()
+------
+
+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.
+
+On Py2 with ``future`` installed, the :mod:`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 :mod:`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 :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 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''