SymPy got this Debian bug report with a patch attached:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=459076

and the same code is in the latest pyglet. I don't understand at all, what the problem is about though - but clearly it was not working for the bug reported and the attached fix fixed that. Is this a bug in pyglet? If not, how can I fix it?

Feel free to ask more questions, I'll forward them to the Debian BTS for the reporter to respond.

Thanks, fixed in r1571.

I found this documentation:

http://tronche.com/gui/x/xlib/window-information/XInternAtom.html

so the problem only happened sometimes?

Yes, atoms are named properties that can be registered with X by any application. The _NET_WM_NAME name is used by most recent window managers, and so pyglet could get away with assuming it was already registered before using it.

The OP is using a window manager that doesn't use this name, and the bug in pyglet was not registering it when it doesn't exist (now fixed). This was just a typo -- all other uses of XInternAtom correctly register the name if it doesn't exist.

I see, thanks for the explanation.