eryksun, thank you for the explanation, the penny finally dropped for me.  It's the *other* child processes that keep the file open, not the parent.  Actually this bug started hitting me after I *stopped* doing exactly what you suggest (fdopen on the temp file), and instead started using file names generated by my program (opened with regular "open") and now I see why that is.  If I use os.fdopen with the O_NOINHERIT flag, it works as expected.