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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''
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