Messages from udev to generic userspace programs are also expected to come from either uid 0 or else something relating to user namespaces which I don't fully understand. I also don't understand why it is necessary for random programs to check this, since netlink normally only allows uid 0 to send messages.
// First, we acquire a reader that gives an exclusive lock
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So the XML package in the standard library, when you imported it, so in its dunder init.py, it would try to import _xmlplus, which was not installed by default, but you could install _xmlplus.