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I've just tested this on OSX 10.8.2 and the problem is not present there (that is, 'python -c "import deadline" &' does not hang)
The problem is present on OSX 10.6.8, and when using ksh(1) you can see why the process is stopped:
[1] + Stopped(SIGTTOU) python -c 'import readline' &
That signal is generated when the program generates output to a tty and the TOSTOP option of stty is active. Whether or not that option is active is OS configuration (and can be changed using the stty command).
Reading the option:
import termios
lflags = termios.tcgetattr(0)[3]
print (lflags & termios.TOSTOP) != 0
Oddly enough the option off on both OSX 10.6 and 10.8, which point to a bug in OSX 10.6 (or to a misunderstanding of low-level tty programming details on my part which would not be unlikely)
Anyway, there is an easy workaround: before importing readline add:
import signal
signal.signal(signal.SIGTTOU, signal.SIG_IGN)
This ignores the SIGTTOU option and allows me to import readline on the background.
IMHO This issue can be closed because this behavior is not a Python bug but a platform feature.