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Tim - at a minimum this should be documented; even if it's
just a link to the ANSI C documentation.  Python is not ANSI
C, we shouldn't expect the Python user to seek out ANSI C 
documentation.  Want me to open a separate doc bug?  The
current doc only says this (about the file builtin):
"""
Modes 'r+', 'w+' and 'a+' open the file for updating (note
that 'w+' truncates the file). Append 'b' to the mode to
open the file in binary mode, on systems that differentiate
between binary and text files (else it is ignored). If the
file cannot be opened, IOError is raised.
"""

Either here, or perhaps in section 2.3.9, a clear
description should be given of how to properly operate a +
mode file.  Failing that, a pointer to ANSI C documentation
so the user can read about it on its own (and so the user
knows that this behavior conforms to the underlying platform
API in every ugly detail).

I'm also dubious that this exposed memory is innocuous, but
I'll defer to your expertise on that one.