Introduction to python module: Ordered Dict (OrderedDict)

Ordered Dictionary-Introduction to OrderedDict

Example

An ordered dictionary is similar to a normal dictionary, except that it can record the order in which elements are inserted into it, while a normal dictionary will iterate in any order. See example below:

import collections

print 'Regular dictionary:'
d = {}
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'
d['d'] = 'D'
d['e'] = 'E'

for k, v in d.items():
  print k, v

print '\nOrderedDict:'
d = collections.OrderedDict()
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'
d['d'] = 'D'
d['e'] = 'E'

for k, v in d.items():
  print k, v

The running results are as follows:

-> python test7.py
Regular dictionary:
a A
c C
b B
e E
d D

OrderedDict:
a A
b B
c C
d D
e E

You can see that usually dictionaries are not traversed in insertion order.

Equality

To determine whether two ordered fields are equal (==), you need to consider whether the order of element insertion is equal

import collections

print 'dict    :',
d1 = {}
d1['a'] = 'A'
d1['b'] = 'B'
d1['c'] = 'C'
d1['d'] = 'D'
d1['e'] = 'E'

d2 = {}
d2['e'] = 'E'
d2['d'] = 'D'
d2['c'] = 'C'
d2['b'] = 'B'
d2['a'] = 'A'

print d1 == d2

print 'OrderedDict:',

d1 = collections.OrderedDict()
d1['a'] = 'A'
d1['b'] = 'B'
d1['c'] = 'C'
d1['d'] = 'D'
d1['e'] = 'E'

d2 = collections.OrderedDict()
d2['e'] = 'E'
d2['d'] = 'D'
d2['c'] = 'C'
d2['b'] = 'B'
d2['a'] = 'A'

print d1 == d2

The running results are as follows:

-> python test7.py
dict    : True
OrderedDict: False

When judging whether an ordered dictionary is equal to other ordinary dictionaries, you only need to judge whether the contents are equal.

Attention

Although the constructor or update() method of OrderedDict accepts keyword parameters, because python function calls use unordered dictionaries to pass parameters, the order of keyword parameters will be lost, so the created ordered dictionary cannot be guaranteed. its order.

References

https://docs.python.org/2/library/collections.html#collections.OrderedDict
https://pymotw.com/2/collections/ordereddict.html