Thread by @raymondh

#Python factlet: The dict.popitem() method is guaranteed to remove key/value pairs in LIFO order.

>>> d = dict(red=1, green=2, blue=3)
>>> d.popitem()
('blue', 3)
>>> d.popitem()
('green', 2)
>>> d.popitem()
('red', 1)

1/

In contrast, OrderedDict.popitem() supports both FIFO and LIFO extraction of key/value pairs.

>>> from collections import OrderedDict
>>> d = OrderedDict(red=1, green=2, blue=3)

>>> d.popitem(last=False) # FIFO
('red', 1)

>>> d.popitem() # LIFO
('blue', 3)

2/

OrderedDict can efficiently move entries to either end without a hash table update.

>>> d = OrderedDict(red=1, green=2, blue=3)

>>> d.move_to_end('green')
>>> list(d)
['red', 'blue', 'green']

>>> d.move_to_end('green', last=False)
>>> list(d)
['green', 'red', 'blue']

3/

These API differences reflect the capabilities of the underlying implementations.

dict() keeps keys in space-efficient sequence, so only right-side appends and pops are fast.

OrderedDict uses a regular dict plus a doubly-linked list, the extra operations are fast.

4/