字典列表中键的顺序

Question

#!/usr/bin/python
# 1.15. Grouping Records Together Based on a Field
# Problem: You have a sequence of dictionaries or instances and you want to iterate over the data
# in groups based on the value of a particular field, such as date.

from operator import itemgetter
from itertools import groupby

# To iterate over the data in chunks grouped by date. 
# First, sort by the desired field (in this case, date) and 
# then use itertools.groupby():

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'},
]

# Sort by the desired field first
rows.sort(key=itemgetter('date'))
print (rows)


for date, items in groupby(rows, key=itemgetter('date')):
    print(date)
    for i in items:
        print(' ', i)

上面代码的输出如下:

[{'date': '07/01/2012', 'address': '5412 N CLARK'}, {'date': '07/01/2012', 'address': '4801 N BROADWAY'}, {'date': '07/02/2012', 'address': '5800 E 58TH'}, {'date': '07/02/2012', 'address': '5645 N RAVENSWOOD'}, {'date': '07/02/2012', 'address': '1060 W ADDISON'}, {'date': '07/03/2012', 'address': '2122 N CLARK'}, {'date': '07/04/2012', 'address': '5148 N CLARK'}, {'date': '07/04/2012', 'address': '1039 W GRANVILLE'}]
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'}

"日期"位于"地址"的前面.但是,如果我通过print (rows)在第24行添加如下更改代码:

#!/usr/bin/python
# 1.15. Grouping Records Together Based on a Field
# Problem: You have a sequence of dictionaries or instances and you want to iterate over the data
# in groups based on the value of a particular field, such as date.

from operator import itemgetter
from itertools import groupby

# To iterate over the data in chunks grouped by date. 
# First, sort by the desired field (in this case, date) and 
# then use itertools.groupby():

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'},
]

print (rows)
# Sort by the desired field first
rows.sort(key=itemgetter('date'))
print (rows)


for date, items in groupby(rows, key=itemgetter('date')):
    print(date)
    for i in items:
        print(' ', i)

上面代码的输出如下:

[{'address': '5412 N CLARK', 'date': '07/01/2012'}, {'address': '4801 N BROADWAY', 'date': '07/01/2012'}, {'address': '5800 E 58TH', 'date': '07/02/2012'}, {'address': '5645 N RAVENSWOOD', 'date': '07/02/2012'}, {'address': '1060 W ADDISON', 'date': '07/02/2012'}, {'address': '2122 N CLARK', 'date': '07/03/2012'}, {'address': '5148 N CLARK', 'date': '07/04/2012'}, {'address': '1039 W GRANVILLE', 'date': '07/04/2012'}]
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'}

"地址"在"日期"前面.

为什么键的顺序会改变？

Answer 1

顺序的变化不是因为你添加了一行代码,而是因为哈希随机化.实现散列随机化使用数万个值的破坏序列来缓解DoS攻击,这些值在例如HTTP POST请求中散列到相同的值.