The problem may occur just because of the way file handles work with
child processes. I'm not sure if it's a bug, or even if it's specific to
Python, so I'm not going to raise another issue. It's worth searching to
see if there's an existing issue about it, though.

Anyone can raise a new bug; you just log in and click on "Create New"
under "Issues" in the menu on the left of the page. I would suggest that
you research the issue first to see if it's previously been raised on
this tracker, or in c.l.py, or on the wider Internet.