OrderedDict in Python

Since Python 3.7, a normal dictionary keeps keys in the order you added them. So you rarely need OrderedDict just to keep order. It is still useful for its extra methods, which let you move keys to either end.

The program

from collections import OrderedDict

steps = OrderedDict()
steps["wash"] = 10
steps["chop"] = 5
steps["cook"] = 20

for name, minutes in steps.items():
    print(name, minutes)

steps.move_to_end("wash")
print(list(steps))

steps.move_to_end("cook", last=False)
print(list(steps))

print(steps.popitem(last=False))
print(list(steps))

wash 10
chop 5
cook 20
['chop', 'cook', 'wash']
['cook', 'chop', 'wash']
('cook', 20)
['chop', 'wash']

How it works

• Items come out in the order they were added, the same as a normal dictionary.
• move_to_end(key) moves a key to the end. With last=False, it moves the key to the start.
• popitem(last=False) removes and returns the first item. A normal dictionary's popitem() can only remove the last one.
• These two methods make OrderedDict a simple way to build a queue of keys, or a cache that removes the oldest entry first.

Order and equality

from collections import OrderedDict

a = {"x": 1, "y": 2}
b = {"y": 2, "x": 1}
print(a == b)

oa = OrderedDict(a)
ob = OrderedDict(b)
print(oa == ob)

True
False

Two normal dictionaries are equal when they have the same keys and values, in any order. Two OrderedDict objects are equal only if the order is also the same.