mirror of https://github.com/python/cpython
Fix entries pertaining to file I/O
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@ -251,17 +251,6 @@ Python 2, but ``b'a' + 'b'`` in Python 3 is a :exc:`TypeError`. A similar issue
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also comes about when doing comparisons between bytes and strings.
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:mod:`io` Module
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''''''''''''''''
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The built-in ``open()`` function in Python 2 always returns a Python 2 string,
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not a unicode string. This is problematic as Python 3's :func:`open` returns a
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string if a file is not opened as binary and bytes if it is.
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To help with compatibility, use :func:`io.open` instead of the built-in
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``open()``. Since :func:`io.open` is essentially the same function in both
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Python 2 and Python 3 it will help iron out any issues that might arise.
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Handle Common "Gotchas"
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-----------------------
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There are a few things that just consistently come up as sticking points for
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@ -269,6 +258,34 @@ people which 2to3 cannot handle automatically or can easily be done in Python 2
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to help modernize your code.
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Specify when opening a file as binary
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'''''''''''''''''''''''''''''''''''''
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Unless you have been working on Windows, there is a chance you have not always
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bothered to add the ``b`` mode when opening a binary file (e.g., ``rb`` for
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binary reading). Under Python 3, binary files and text files are clearly
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distinct and mutually incompatible; see the :mod:`io` module for details.
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Therefore, you **must** make a decision of whether a file will be used for
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binary access (allowing to read and/or write bytes data) or text access
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(allowing to read and/or write unicode data).
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Text files
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''''''''''
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Text files created using ``open()`` under Python 2 return byte strings,
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while under Python 3 they return unicode strings. Depending on your porting
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strategy, this can be an issue.
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If you want text files to return unicode strings in Python 2, you have two
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possibilities:
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* Under Python 2.6 and higher, use :func:`io.open`. Since :func:`io.open`
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is essentially the same function in both Python 2 and Python 3, it will
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help iron out any issues that might arise.
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* If pre-2.6 compatibility is needed, then you should use :func:`codecs.open`
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instead. This will make sure that you get back unicode strings in Python 2.
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Subclass ``object``
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'''''''''''''''''''
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New-style classes have been around since Python 2.2. You need to make sure you
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@ -392,23 +409,9 @@ http://lucumr.pocoo.org/2011/1/22/forwards-compatible-python/)::
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return u'spam-spam-bacon-spam' # 2to3 will remove the 'u' prefix
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Specify when opening a file as binary
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'''''''''''''''''''''''''''''''''''''
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Unless you have been working on Windows, there is a chance you have not always
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bothered to add the ``b`` mode when opening a file (e.g., ``
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Use :func:``codecs.open()``
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'''''''''''''''''''''''''''
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If you are not able to limit your Python 2 compatibility to 2.6 or newer (and
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thus get to use :func:`io.open`), then you should make sure you use
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:func:`codecs.open` over the built-in ``open()`` function. This will make sure
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that you get back unicode strings in Python 2 when reading in text and an
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instance of ``str`` when dealing with bytes.
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Don't Index on Exceptions
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'''''''''''''''''''''''''
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In Python 2, the following worked::
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>>> exc = Exception(1, 2, 3)
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@ -423,9 +426,9 @@ sequence containing all arguments passed to the :meth:`__init__` method.
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Even better is to use documented attributes the exception provides.
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Don't use ``__getslice__`` & Friends
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''''''''''''''''''''''''''''''''''''
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Been deprecated for a while, but Python 3 finally drops support for
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``__getslice__()``, etc. Move completely over to :meth:`__getitem__` and
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friends.
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