#implements Bag ADT container using a Python list structure

class Bag:

def __init__(self):

self._theItems = list()

def __len__(self):

return len(self._theItems)

def __contains__(self, item):

return item in self._theItems

def add(self, item):

self._theItems.append(item)

def remove(self, item):

assert item in self._theItems, "The item must be in the bag."

ndx = self._theItems.index(item)

return self._theItems.pop(ndx)

def __iter__(self):

return _BagIterator(self._theItems)

class _BagIterator:

def __init__(self, theList):

self._bagItems = theList

self._curItem = 0

def __iter__(self):

return self

def __next__(self):

if self._curItem < len(self._bagItems):

item = self._bagItems[self._curItem]

self._curItem += 1

return item

else:

raise StopIteration