an instance of a dict subclass, mutable

remembers the order that keys were first inserted

from collections import OrderedDict

d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}
o = OrderedDict(d) # OrderedDict([('banana', 3), ('apple', 4), ('pear', 1), ('orange', 2)])

# insert item
o['pineapple'] = 10 # OrderedDict([('banana', 3), ('apple', 4), ('pear', 1), ('orange', 2), ('pineapple', 10)])
            

o['apple'] # 4

list(o.keys()) # ['banana', 'apple', 'pear', 'orange', 'pineapple']
list(o.values()) # [3, 4, 1, 2, 10]
list(o.items()) # [('banana', 3), ('apple', 4), ('pear', 1), ('orange', 2), ('pineapple', 10)]

# update
o['apple'] = 10

# empty
#o.clear()
            

d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}
o = OrderedDict(d) # OrderedDict([('banana', 3), ('apple', 4), ('pear', 1), ('orange', 2)])

item = o.popitem(last = True) # default, return the last item in LIFO order
print(o, item) # OrderedDict([('banana', 3), ('apple', 4), ('pear', 1)]) ('orange', 2)
# item = o.popitem(last = False) # return the first item in FIFO order
# print(o, item) # OrderedDict([('OrderedDict([('OrderedDict([('OrderedDict([('apple', 4), ('pear', 1), ('orange', 2)]) ('banana', 3)
            

d = {'banana': 3, 'apple': 4, 'pear': 1, 'orange': 2}
o = OrderedDict(d) # OrderedDict([('banana', 3), ('apple', 4), ('pear', 1), ('orange', 2)])

o.move_to_end('apple', last = True) # default, the item is moved to the right end 
print(o) # OrderedDict([('banana', 3), ('pear', 1), ('orange', 2), ('apple', 4)])
# o.move_to_end('apple', last = False) # default, the item is moved to the left end
# print(o) # OrderedDict([('apple', 4), ('banana', 3), ('pear', 1), ('orange', 2)])
            

len(o) # 4
list(o) # ['banana', 'apple', 'pear', 'orange']
            

Python 3 Standard Library

Python 3 Module of the Week