An OrderedDict is a dictionary subclass that remembers the order in which its contents are added.



In [1]:

    
import collections

print('Regular dictionary:')
d = {}
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'

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

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

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









    



Regular dictionary:
a A
b B
c C

OrderedDict:
a A
b B
c C

A regular dict does not track the insertion order, and iterating over it produces the values in order based on how the keys are stored in the hash table, which is in turn influenced by a random value to reduce collisions. In an OrderedDict, by contrast, the order in which the items are inserted is remembered and used when creating an iterator.

Equality

A regular dict looks at its contents when testing for equality. An OrderedDict also considers the order in which the items were added.



In [2]:

    
import collections

print('dict       :', end=' ')
d1 = {}
d1['a'] = 'A'
d1['b'] = 'B'
d1['c'] = 'C'

d2 = {}
d2['c'] = 'C'
d2['b'] = 'B'
d2['a'] = 'A'

print(d1 == d2)

print('OrderedDict:', end=' ')

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

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

print(d1 == d2)









    



dict       : True
OrderedDict: False

In this case, since the two ordered dictionaries are created from values in a different order, they are considered to be different.

Reordering

It is possible to change the order of the keys in an OrderedDict by moving them to either the beginning or the end of the sequence using move_to_end().



In [3]:

    
import collections

d = collections.OrderedDict(
    [('a', 'A'), ('b', 'B'), ('c', 'C')]
)

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

d.move_to_end('b')

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

d.move_to_end('b', last=False)

print('\nmove_to_end(last=False):')
for k, v in d.items():
    print(k, v)









    



Before:
a A
b B
c C

move_to_end():
a A
c C
b B

move_to_end(last=False):
b B
a A
c C



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