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)

以上代码的输出如下：

[{'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”在“地址”前面。 但是，如果我通过在第24行添加print (rows)来更改代码，如下所示：

#!/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)

以上代码的输出如下：

[{'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。你知道吗

为什么钥匙的顺序会改变？你知道吗

Answer 1

如果希望顺序保持不变，则需要使用来自collections的OrderedDict。你知道吗

from collections import OrderedDict

row = OrderedDict([('address', '5412 N CLARK'), ('date', '07/01/2012')])

>>> row
OrderedDict([('address', '5412 N CLARK'), ('date', '07/01/2012')])

>>> rows.keys()
['address', 'date']

Answer 2

顺序的变化不是因为添加了一行代码，而是因为hash randomization。实现散列随机化可使用数万个值的断开序列缓解DoS攻击，这些值在HTTP POST请求中散列为相同的值。你知道吗