The 'radio' option to the Tix.CheckList widget cannot
be set.  Attempting to change the option during widget
creation results in the following error:

_tkinter.TclError: cannot assigned to static variable
"-radio"

The radio option is declared static in the Tix tcl
library, and must be set at widget creation time.  The
radio option is not included in the list of static
options passed to TixWidget.__init__ from
CheckList.__init__ in file Tix.py. 

Solution:

Add 'radio' to the list of static options passed to
TixWidget.__init__ in Tix.py.  Output from diff -C 1:

*** 1562,1564 ****
          TixWidget.__init__(self, master, 'tixCheckList',
!                            ['options'], cnf, kw)
          self.subwidget_list['hlist'] =
_dummyHList(self, 'hlist')
--- 1562,1564 ----
          TixWidget.__init__(self, master, 'tixCheckList',
!                            ['options','radio'], cnf, kw)
          self.subwidget_list['hlist'] =
_dummyHList(self, 'hlist')