Add create mode to open()

Doc/library/io.rst

 :mod:`io` --- Core tools for working with streams
 =================================================
 
 .. module:: io
    :synopsis: Core tools for working with streams.
 .. moduleauthor:: Guido van Rossum <guido@python.org>
 .. moduleauthor:: Mike Verdone <mike.verdone@gmail.com>
 .. moduleauthor:: Mark Russell <mark.russell@zen.co.uk>
 .. moduleauthor:: Antoine Pitrou <solipsis@pitrou.net>
 .. moduleauthor:: Amaury Forgeot d'Arc <amauryfa@gmail.com>
@@ skipping 457 matching lines @@
    It implements the :class:`RawIOBase` interface (and therefore the
    :class:`IOBase` interface, too).
 
    The *name* can be one of two things:
 
    * a character string or bytes object representing the path to the file
      which will be opened;
    * an integer representing the number of an existing OS-level file descriptor
      to which the resulting :class:`FileIO` object will give access.
 
-   The *mode* can be ``'r'``, ``'w'`` or ``'a'`` for reading (default), writing,
-   or appending.  The file will be created if it doesn't exist when opened for
-   writing or appending; it will be truncated when opened for writing.  Add a
-   ``'+'`` to the mode to allow simultaneous reading and writing.
+   The *mode* can be ``'x'``, ``'r'``, ``'w'`` or ``'a'`` for creating,
+   reading (default), writing, or appending.  The file will be created if it 
+   doesn't exist when opened for writing or appending; it will be 
+   truncated when opened for writing.  An IOError will be raise if it

[Benjamin Peterson, 2012/01/08 16:16:10]

Actually, :exc:`OSError`.

+   already exists when opened for creating. Opening a file for creating 
+   implies writing, so this mode behaves in a similar way to ``'w'``.  
+   Add a ``'+'`` to the mode to allow simultaneous reading and writing.
 
    The :meth:`read` (when called with a positive argument), :meth:`readinto`
    and :meth:`write` methods on this class will only make one system call.
 
    In addition to the attributes and methods from :class:`IOBase` and
    :class:`RawIOBase`, :class:`FileIO` provides the following data
    attributes and methods:
 
    .. attribute:: mode
 
@@ skipping 354 matching lines @@
 they can arise from doing I/O in a :mod:`signal` handler.  If a thread tries to
 renter a buffered object which it is already accessing, a :exc:`RuntimeError` is
 raised.  Note this doesn't prohibit a different thread from entering the
 buffered object.
 
 The above implicitly extends to text files, since the :func:`open()` function
 will wrap a buffered object inside a :class:`TextIOWrapper`.  This includes
 standard streams and therefore affects the built-in function :func:`print()` as
 well.