问题：想根据字典或者对象实例的某个特定的字典（比如日期）来分组迭代数据
解决方案：itertools.groupby()函数在对数据进行分组时特别有用（前提是先以目标字典进行排序）
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
rows.sort(key=itemgetter('date')) #首先以date字段进行排序
for date, items in groupby(rows, key=itemgetter('date')): #再以date进行分组
print(date)
for i in items:
print(' ', i)
#defaultdict构建一个一键多值的字典会更好
#Example of building a multidict
from collections import defaultdict
rows_by_date = defaultdict(list) #创建一个一键多值的字典，
for row in rows:
rows_by_date[row['date']].append(row)
for r in rows_by_date['07/01/2012']:
print(r)
>>> ================================ RESTART ================================
>>>
07/01/2012
{'address': '5412 N CLARK', 'date': '07/01/2012'}
{'address': '4801 N BROADWAY', 'date': '07/01/2012'}
07/02/2012
{'address': '5800 E 58TH', 'date': '07/02/2012'}
{'address': '5645 N RAVENSWOOD', 'date': '07/02/2012'}
{'address': '1060 W ADDISON', 'date': '07/02/2012'}
07/03/2012
{'address': '2122 N CLARK', 'date': '07/03/2012'}
07/04/2012
{'address': '5148 N CLARK', 'date': '07/04/2012'}
{'address': '1039 W GRANVILLE', 'date': '07/04/2012'}
{'address': '5412 N CLARK', 'date': '07/01/2012'}
{'address': '4801 N BROADWAY', 'date': '07/01/2012'}
>>>
