OrderedDict in Python

Ordered dictionaries are just like regular dictionaries but have some extra capabilities relating to ordering operations. Both ordered dictionary and the built-in dict can remember insertion order. Some differences from dict are:

Regular dict is efficient at mapping operations, but tracking insertion order is secondary. OrderedDict in Python is efficient in reordering operations, but iteration speed, and the performance of update operations is secondary.
Equality operation for OrderedDict checks for matching order.
popitem() method of OrderedDict has a different signature.

Syntax

To create ordered dictionary, syntax is

collections.OrderedDict([items])

It return an instance of a dict subclass that has methods specialized for rearranging dictionary order.

Important Methods

OrderedDict supports usual mapping methods.

popitem(last=True) : Returns and removes a (key, value) pair. The pairs are returned in LIFO order if last is true or FIFO order if false.
move_to_end(key, last=True) : Move an existing key to either end of an ordered dictionary. If last is true, item is moved to the right end else in the beginning.

Example

Below example creates two ordered dictionary. First dictionary is created by adding key and value, while other dictionary is created using list. To get the winning team, popitem() is used.

# Demonstrate the usage of OrderedDict objects
from collections import OrderedDict

# 1. Create OrderedDict
d = OrderedDict()
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'

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

# Output
# a A
# b B
# c C

# Teams with scores
sportTeams = [("Royals", 18), ("Rockets", 24), ("Dragons", 22),
            ("Kings", 15), ("Jets", 16), ("Warriors", 25)]

# Sort the teams
sortedTeams = sorted(sportTeams, key=lambda t: t[1], reverse=True)

# 2. Create an ordered dictionary of the teams
teams = OrderedDict(sortedTeams)
print(teams)

# Output
# OrderedDict([('Warriors', 25), ('Rockets', 24), ('Dragons', 22), ('Royals', 18), ('Jets', 16), ('Kings', 15)])

# Remove the top item
tm, wl = teams.popitem(False)
print("Top team: ", tm, wl)

# Output
# Top team:  Warriors 25

Reverse Order

OrderedDict also support reverse iterator, which is not supported in regular dictionary. Below example get the keys of dictionary in reverse order and traverse using for loop. reversed() return a reverse iterator.

# Demonstrate the usage of OrderedDict objects
from collections import OrderedDict

# 1. Create OrderedDict
d = OrderedDict()
d['a'] = 'A'
d['b'] = 'B'
d['c'] = 'C'

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

# Output
# a A
# b B
# c C

itr = reversed(d)

for k in itr:
    print(k, d[k])

# Output
# c C
# b B
# a A

Reordering

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

import collections

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

print(d)

# Output : Initial Order
# OrderedDict([('a', 'A'), ('b', 'B'), ('c', 'C')])
d.move_to_end('b')

print(d)

# Output : Order after move_to_end
# OrderedDict([('a', 'A'), ('c', 'C'), ('b', 'B')])

Equality

A regular dict looks at its contents when testing for equality. An OrderedDict also considers the order in which the items were added. Equality tests between OrderedDict objects and other Mapping objects are order-insensitive like regular dictionaries. So OrderedDict objects can be substituted anywhere a regular dictionary is used.

import collections

d1 = collections.OrderedDict([('a', 'A'), ('b', 'B'), ('c', 'C')])
d2 = collections.OrderedDict([('b', 'B'), ('a', 'A'), ('c', 'C')])
d3 = collections.OrderedDict([('a', 'A'), ('b', 'B'), ('c', 'C')])

d4 = {}
d4['a'] = 'A'
d4['b'] = 'B'
d4['c'] = 'C'

d5 = {}
d5['b'] = 'B'
d5['c'] = 'C'
d5['a'] = 'A'

# Ordered Dict equality
print(d1 == d2)
# Output
# False

print(d1 == d3)
# Output
# True

# Regular Dict equality
print(d4 == d5)
# Output
# True

print(d1 == d5)
# Output
# True