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I think Cory was aware of the underlying requirement to
interpose a seek operation between the write and read
operations, but was concerned about the consequences of not
doing so.  Python normally protects one from doing things
that are *too* dangerous: I guess it's unclear to him (and
perhaps others) whether the current behavior of file is just
(relatively) obscure or if it could lead to real problems
(exposing sensitive data, crashing the process).  It seems
like the latter is somewhat unlikely in practice, but since
the behavior is unspecified, it seems like it *could* happen.

I guess since Tim closed this, he thinks it's not too
dangerous.  In this case, the documentation could probably
stand to be improved somewhat.  The section
(<http://python.org/doc/lib/built-in-funcs.html#built-in-funcs>)
that documents the various modes which can be used to open a
file could updated to include a warning along the lines of
that in the C standard.  It should probably explicitly state
which file methods can be used to satisfy this requirement,
since it's not clear otherwise except by reading the
implementation of the file type (one could guess, from
<http://python.org/doc/lib/bltin-file-objects.html#bltin-file-objects>
that file.flush() and file.seek() are suitable, but the
documentation for these only says "like stdio ...", so you
can't be completely sure).