It is sys.exit() not os.exit()!

tools/task_plots/iplot_tasks.py

@@ -402,7 +402,7 @@ data = data[1:, :]
 # Exit if no data.
 if data.size == 0:
     print(("# Rank ", rank, " has no tasks"))
-    os.exit(1)
+    sys.exit(1)
 
 start_t = float(tic_step)
 data[:, ticcol] -= start_t

[Josh Borrow]

There's also the builtin exit() that does not require either sys or os.

[Josh Borrow]

(I just learned that sys.exit is better practice than exit, so carry on :p)