OrderedDict

Learn ordered dicts with OrderedDict.

OrderedDict

OrderedDict is a dictionary type that keeps the insertion order of keys. In Python 3.7+ the standard dict also keeps order, but OrderedDict offers extra methods.

The most important extra method is move_to_end(key, last=True). It moves a key to the end (default) or to the beginning. popitem(last=True) removes the last or first item. These methods are useful for cache or ordered-queue applications.

The move_to_end method

With move_to_end you move a key to the end or beginning of the dict. By default the key goes to the end; with last=False it moves to the beginning. This is often used in LRU (least recently used) cache structures.

Example: od = OrderedDict()\nod["a"] = 1\nod["b"] = 2\nod.move_to_end("a"). After this code the order is b, a. With list(od.keys()) you can see the keys as an ordered list.

Exercise Reorder keys with OrderedDict

Start with from collections import OrderedDict. Create an OrderedDict with od = OrderedDict(). Add two keys with od["a"] = 1 and od["b"] = 2. Move a to the end with od.move_to_end("a"). Then print the keys with print(list(od.keys())).

Show example solution

# Reorder keys with OrderedDict
from collections import OrderedDict

od = OrderedDict()
od["a"] = 1
od["b"] = 2
od.move_to_end("a")
print(list(od.keys()))

Test yourself

Which module does OrderedDict come from?
- functools
- typing
- itertools
- collections
Show answer

collections — OrderedDict is in the collections module.

move_to_end call

od = OrderedDict()
od["a"] = 1
od["b"] = 2
od.____("a")
print(list(od.keys()))

Show answer

od = OrderedDict()
od["a"] = 1
od["b"] = 2
od.move_to_end("a")
print(list(od.keys()))

Match the OrderedDict parts

OrderedDict

Ordered dict
move_to_end

Moves a key
popitem

Removes an item
collections

Module

Next lesson: deque

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