OrderedDict in python

The dict type in python is not ordered, and it's been a pain, especially when you want to keep the sorted order inside a dictionary. From python 2.7, OrderedDict is supported. This quick example will show you what are the differences.

from collections import *

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

OrderedItem() keeps the input order, with this you can sort

Regular dictionary:
a A
c C
b B
e E
d D

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

• OrderedDict() requires import collections.
• When printing the object, OrderedDict() doesn't show it with '{' and '}'.

You can build OrderedDict() using list (iteration), and you can keep the order now.

d = OrderedDict([('apple', 4), ('banana', 3), ('orange', 2), ('pear', 1)])
for k, v in d.items():
    print k, v

apple 4
banana 3
orange 2
pear 1

Using this, you can get the sorted dictionary. Remembering dict.items() return list, you can sort regular dictionary.

# dictionary sorted by value
d = {'banana': 3, 'apple':4, 'pear': 1, 'orange': 2}
d = OrderedDict(sorted(d.items(), key=lambda t: t[0]))
for k, v in d.items():
    print k, v
d = OrderedDict(sorted(d.items(), key=lambda t: t[1]))
for k, v in d.items():
    print k, v

apple 4
banana 3
orange 2
pear 1

pear 1
orange 2
banana 3
apple 4

sort() method gets a list as an input and returns sorted list, it doesn't modify the original list.

a = [1,2,3,4,-3]
b = sorted(a)
print b
print a

[-3, 1, 2, 3, 4]
[1, 2, 3, 4, -3]

Even you can sort by the length of the index.

d = {'banana': 3, 'apple':4, 'pear': 1, 'orange': 2}
# dictionary sorted by length of the key string
d = OrderedDict(sorted(d.items(), key=lambda t: len(t[0])))
for k, v in d.items():
    print k, v

pear 1
apple 4
orange 2
banana 3

You can get the dict from OrderedDict() as the latter is iterable.

o = OrderedDict([('pear', 1), ('apple', 4), ('orange', 2), ('banana', 3)])
d = dict(o)
for k, v in d.items():
    print k, v

orange 2
pear 1
apple 4
banana 3

References

• Python 2.7 documentation on OrderedDict()
• Doug Hellmann's OrderedDict()