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I wrote in an earlier message that a file opened with O_TEMPORARY must be reopened with O_TEMPORARY. This is not quite accurate.
Using O_TEMPORARY causes the FILE_SHARE_DELETE sharing mode to be used, and a file currently opened with FILE_SHARE_DELETE can only be reopened with FILE_SHARE_DELETE.
Unfortunately using O_TEMPORARY is the only way allowed by msvcrt to get FILE_SHARE_DELETE, even though it also has the orthogonal effect of unlinking the file when all handles are closed.
The nice thing about FILE_SHARE_DELETE is that it gives Unix-like behaviour: the file can be renamed or deleted while you have an open handle, and you can still continue to use the handle.
Attached is a mostly untested attempt at writing replacements for open() and os.open() which use the FILE_SHARE_DELETE sharing mode. Among other things, these can be used for reopening temporary files.
Even if tempfile does not use make use of this, I think something similar would be useful in the stdlib.