Confused over Lists

[Michael Chermside]

Mark McEahern wrote:
> Did I miss it or did you omit the code for MovableListIterator itself?

Apparently I failed to post the code itself. Here it is, along with the 
test code:

class MovableListIterator:
     """This is an iterator for lists which allows one to advance
     or regress the position in the list. It also will raise an
     exception if the list is lengthened or shortened."""

     def __init__(self, myList):
         self.myList = myList
         self.originalLength = len(myList)
         self.nextIndex = 0

     def __iter__(self):
         return self

     class ListModifiedException(Exception):
         pass

     def next(self):
         if len(self.myList) != self.originalLength:
             raise MovableListIterator.ListModifiedException

         index = self.nextIndex
         if index >= self.originalLength:
             raise StopIteration
         self.nextIndex += 1
         return self.myList[index]

     def advance(self, places=1):
         self.nextIndex += places

     def regress(self, places=1):
         self.nextIndex -= places




import unittest
class TestMovableListIterator(unittest.TestCase):
     def test_create(self):
         li = list('abcde')
         i = MovableListIterator(li)
     def test_basic_loop(self):
         li = 'abcde'
         self.assertEqual( list(li), [x for x in MovableListIterator(li)] )
     def test_advance(self):
         li = 'abcdef'
         result = []
         i = MovableListIterator(li)
         result.append( i.next() )
         i.advance()
         result.append( i.next() )
         i.advance(2)
         result.append( i.next() )
         self.assertEqual( list('acf'), result )
         self.assertRaises( StopIteration, i.next )
     def test_regress(self):
         li = 'abc'
         result = []
         i = MovableListIterator(li)
         result.append( i.next() )
         i.regress()
         result.append( i.next() )
         result.append( i.next() )
         result.append( i.next() )
         i.regress(2)
         result.append( i.next() )
         result.append( i.next() )
         self.assertEqual( list('aabcbc'), result )
         self.assertRaises( StopIteration, i.next )
     def test_shrink_throws(self):
         li = [1,2,3,4]
         i = MovableListIterator(li)
         li.pop()
         self.assertRaises( MovableListIterator.ListModifiedException,
                            i.next )
     def test_grow_throws(self):
         li = [1,2,3,4]
         i = MovableListIterator(li)
         li.append(5)
         self.assertRaises( MovableListIterator.ListModifiedException,
                            i.next )