反復するとき、zipとizipの間に機能的な違いはありますか？

私は以下のようなコードを持っています：

for v1, v2 in zip(iter1, iter2):
   print len(v1) # prints 0

しかし、zipをitertools.izipに変更すると、1が出力されます。

for v1, v2 in izip(iter1, iter2):
   print len(v1) # prints 1

他のすべてのコードは同じです。zipをizipに置き換えるだけで、機能しました。izipの出力は正しいものです。

編集：コード全体の追加：

#!/bin/python

"""
How to use:
>>> from color_assign import Bag, assign_colors
>>> from pprint import pprint
>>> old_topics = set([
... Bag(name='T1', group=0, color=1, count=16000),
... Bag(name='T2', group=0, color=1, count=16000),
... Bag(name='T3', group=1, color=2, count=16000),
... Bag(name='T4', group=2, color=3, count=16000),
... ])
>>> new_topics = set([
... Bag(name='T1', group=0, color=None, count=16000),
... Bag(name='T2', group=4, color=None, count=16000),
... Bag(name='T3', group=1, color=None, count=16000),
... Bag(name='T4', group=1, color=None, count=16000),
... ])
>>> color_ranges = [ [1,10] ]
>>> assign_colors(old_topics, new_topics, color_ranges)
>>> pprint(sorted(new_topics, key=attrgetter('name')))
[Bag(name=T1, group=0, color=1, count=16000),
 Bag(name=T2, group=4, color=3, count=16000),
 Bag(name=T3, group=1, color=2, count=16000),
 Bag(name=T4, group=1, color=2, count=16000)]
>>> 
"""

from itertools import groupby, izip
from operator import attrgetter

class Bag:
  def __init__(self, name, group, color=None, count=None):
    self.name  = name 
    self.group = group
    self.color    = color   
    self.count  = count 
  def __repr__(self):
    return "Bag(name={self.name}, group={self.group}, color={self.color}, count={self.count})".format(self=self)
  def __key(self):
    return self.name
  def __hash__(self):
    return hash(self.__key())
  def __eq__(self, other):
    return type(self) is type(other) and self.__key() == other.__key()

def color_range_gen(color_ranges, used_colors):
  color_ranges = sorted(color_ranges)
  color_iter = iter(sorted(used_colors))
  next_used = next(color_iter, None)
  for start_color, end_color in color_ranges:
    cur_color = start_color
    end_color = end_color
    while cur_color <= end_color:
      if cur_color == next_used:
        next_used = next(color_iter, None)
      else:
        yield cur_color
      cur_color = cur_color + 1


def assign_colors(old_topics, new_topics, color_ranges):
  old_topics -= (old_topics-new_topics) #Remove topics from old_topics which are no longer present in new_topics
  used_colors = set()

  def group_topics(topics):
    by_group = attrgetter('group')
    for _, tgrp in groupby(sorted(topics, key=by_group), by_group):
      yield tgrp

  for topic_group in group_topics(old_topics):
    oldtset = frozenset(topic_group)
    peek = next(iter(oldtset))
    try:
      new_group = next(topic.group for topic in new_topics if topic.name == peek.name and not topic.color)
    except StopIteration:
      continue
    newtset = frozenset(topic for topic in new_topics if topic.group == new_group)
    if oldtset <= newtset:
      for topic in newtset:
        topic.color = peek.color
      used_colors.add(peek.color)

  free_colors = color_range_gen(color_ranges, used_colors)
  unassigned_topics = (t for t in new_topics if not t.color)
  for tset, color in zip(group_topics(unassigned_topics), free_colors):
    for topic in tset:
      topic.color = color

if __name__ == '__main__':
  import doctest
  doctest.testmod()

使用法：

my_host:my_dir$ /tmp/color_assign.py
**********************************************************************
File "/tmp/color_assign.py", line 21, in __main__
Failed example:
    pprint(sorted(new_topics, key=attrgetter('name')))
Expected:
    [Bag(name=T1, group=0, color=1, count=16000),
     Bag(name=T2, group=4, color=3, count=16000),
     Bag(name=T3, group=1, color=2, count=16000),
     Bag(name=T4, group=1, color=2, count=16000)]
Got:
    [Bag(name=T1, group=0, color=None, count=16000),
     Bag(name=T2, group=4, color=3, count=16000),
     Bag(name=T3, group=1, color=2, count=16000),
     Bag(name=T4, group=1, color=2, count=16000)]
**********************************************************************
1 items had failures:
   1 of   7 in __main__
***Test Failed*** 1 failures.
my_host:my_dir$ sed -i 's/zip(/izip(/g' /tmp/color_assign.py
my_host:my_dir$ /tmp/color_assign.py
my_host:my_dir$

更新：問題は、groupbyzipを使用するときにイテレータを無効にすることです。