40 lines
1.5 KiB
ReStructuredText
40 lines
1.5 KiB
ReStructuredText
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.. _open-function:
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open()
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------
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The Python 3 builtin :func:`open` function for opening files returns file
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contents as (unicode) strings unless the binary (``b``) flag is passed, as in::
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open(filename, 'rb')
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in which case its methods like :func:`read` return Py3 :class:`bytes` objects.
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On Py2 with ``future`` installed, the :mod:`builtins` module provides an
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``open`` function that is mostly compatible with that on Python 3 (e.g. it
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offers keyword arguments like ``encoding``). This maps to the ``open`` backport
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available in the standard library :mod:`io` module on Py2.7.
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One difference to be aware of between the Python 3 ``open`` and
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``future.builtins.open`` on Python 2 is that the return types of methods such
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as :func:`read()` from the file object that ``open`` returns are not
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automatically cast from native bytes or unicode strings on Python 2 to the
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corresponding ``future.builtins.bytes`` or ``future.builtins.str`` types. If you
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need the returned data to behave the exactly same way on Py2 as on Py3, you can
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cast it explicitly as follows::
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from __future__ import unicode_literals
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from builtins import open, bytes
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data = open('image.png', 'rb').read()
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# On Py2, data is a standard 8-bit str with loose Unicode coercion.
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# data + u'' would likely raise a UnicodeDecodeError
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data = bytes(data)
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# Now it behaves like a Py3 bytes object...
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assert data[:4] == b'\x89PNG'
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assert data[4] == 13 # integer
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# Raises TypeError:
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# data + u''
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