问题：有一个字典或者实例的序列，想根据某个特定的字段比如date 来分组迭代访问。
answer: itertools.groupby函数对于这样的数据分组操作非常实用
eg:
rows = [
{‘address‘: ‘5412 N CLARK‘, ‘date‘: ‘07/01/2012‘},
{‘address‘: ‘5148 N CLARK‘, ‘date‘: ‘07/04/2012‘},
{‘address‘: ‘5800 E 58TH‘, ‘date‘: ‘07/02/2012‘},
{‘address‘: ‘2122 N CLARK‘, ‘date‘: ‘07/03/2012‘},
{‘address‘: ‘5645 N RAVENSWOOD‘, ‘date‘: ‘07/02/2012‘},
{‘address‘: ‘1060 W ADDISON‘, ‘date‘: ‘07/02/2012‘},
{‘address‘: ‘4801 N BROADWAY‘, ‘date‘: ‘07/01/2012‘},
{‘address‘: ‘1039 W GRANVILLE‘, ‘date‘: ‘07/04/2012‘},
]
>>>from operator import itemgetter
>>>from itertools import groupby
# Sort by the desired field first
>>>rows.sort(key=itemgetter(‘date‘))
# Iterate in groups
>>>for date, items in groupby(rows, key=itemgetter(‘date‘)):
....... print(date)
....... for i in items:
....... print(‘ ‘, i)
运行结果：
07/01/2012
{‘date‘: ‘07/01/2012‘, ‘address‘: ‘5412 N CLARK‘}
{‘date‘: ‘07/01/2012‘, ‘address‘: ‘4801 N BROADWAY‘}
07/02/2012
{‘date‘: ‘07/02/2012‘, ‘address‘: ‘5800 E 58TH‘}
{‘date‘: ‘07/02/2012‘, ‘address‘: ‘5645 N RAVENSWOOD‘}
{‘date‘: ‘07/02/2012‘, ‘address‘: ‘1060 W ADDISON‘}
07/03/2012
{‘date‘: ‘07/03/2012‘, ‘address‘: ‘2122 N CLARK‘}
07/04/2012
{‘date‘: ‘07/04/2012‘, ‘address‘: ‘5148 N CLARK‘}
{‘date‘: ‘07/04/2012‘, ‘address‘: ‘1039 W GRANVILLE‘}
groupby()函数扫描整个序列并且查找连续相同值的元素序列,
在每次迭代的时候它会返回一个值和一个迭代器对象,
这个迭代器对象可以生成元素值全部等于上面那个值的组中所有对象,
要根据指定的字段将数据排序,因为groupby() 仅仅检查连续的元素，如果事先并没有排序完成的话，分组函数将得不到想要的结果。