io.FileIO and io.open should support openat

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 461 matching lines @@
      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 :meth:`read` (when called with a positive argument), :meth:`readinto`
    and :meth:`write` methods on this class will only make one system call.
 
-   A custom opener can be used by passing a callable as *opener*. The
-   underlying file descriptor for the file object is then obtained by calling
-   *opener* with (*name*, *flags*).
+   A custom opener can be used by passing a callable as *opener*. The underlying
+   file descriptor for the file object is then obtained by calling *opener* with
+   (*name*, *flags*). *opener* must return an open file descriptor (passing
+   :mod:`os.open` as *opener* results in functionality similar to passing
+   ``None``).
 
    In addition to the attributes and methods from :class:`IOBase` and
    :class:`RawIOBase`, :class:`FileIO` provides the following data
    attributes and methods:
 
    .. attribute:: mode
 
       The mode as given in the constructor.
 
    .. attribute:: name
@@ skipping 355 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.