[forwarded from https://launchpad.net/bugs/597763]

seen with all version, built with Tcl8.5

To reproduce the bug, use the following class:

class x(object):
 TEST = "hello"
 THE_MAXIMUM = 55

 def __init__(self, a, b):
  self.a = 3
  self.b = "test"

and set the cursor behind "test". After that use SHIFT + UP (so from 3 to incl. "test" will be marked) and after that use SHIFT + POS1, which will produce this exception:

>>> Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python2.6/lib-tk/Tkinter.py", line 1413, in __call__
    return self.func(*args)
  File "/usr/lib/python2.6/idlelib/MultiCall.py", line 151, in handler
    r = l[i](event)
  File "/usr/lib/python2.6/idlelib/EditorWindow.py", line 335, in home_callback
    if self.text.compare(first,">",last):
  File "/usr/lib/python2.6/lib-tk/Tkinter.py", line 2863, in compare
    self._w, 'compare', index1, op, index2))
TclError: expected boolean value but got ""

This issue was fixed in 500e48708470, as part of issue3851.

The problem was originally reported in Issue4676 and fixed partly there and partly in Issue3851.