How to fix the descriptor issue.

descriptor_fix.py

import collections

test = [['A'], ['B'], ['C']]


class ValueDescriptor(object):

    def __init__(self):
        self.value_store = collections.defaultdict(lambda: None)

    def __get__(self, obj, objtype):
        return self.value_store[obj]

    def __set__(self, obj, value):
        self.value_store[obj] = value


class PropItem(object):

    value = None

    def __init__(self, value):
        self.value = value


class DescItem(object):

    value = ValueDescriptor()

    def __init__(self, value):
        self.value = value

prop_results = [PropItem(value=each) for each in test]
desc_results = [DescItem(value=each) for each in test]
print "Expected:", test
print "PropItem:", [each.value for each in prop_results]
print "DescItem:", [each.value for each in desc_results]

# $ python iterfail.py
# Expected: [['A'], ['B'], ['C']]
# PropItem: [['A'], ['B'], ['C']]
# DescItem: [['A'], ['B'], ['C']]