Difference between dictionary and OrderedDict

问题:

I am trying to get a sorted dictionary. But the order of the items between mydict and orddict doesn't seem to change.

from collections import OrderedDict

mydict = {'a': 1, 'b': 2, 'c': 3, 'd': 4}

orddict = OrderedDict(mydict)

print(mydict, orddict)

# print items in mydict:
print('mydict')
for k, v in mydict.items():
    print(k, v)

print('ordereddict')
# print items in ordered dictionary
for k, v in orddict.items():
    print(k, v)


# print the dictionary keys
# for key in mydict.keys():
#     print(key)


#  print the dictionary values
# for value in mydict.values():
#     print(value)

回答1:

An OrderedDict preserves the order elements were inserted:

>>> od = OrderedDict()
>>> od['c'] = 1
>>> od['b'] = 2
>>> od['a'] = 3
>>> od.items()
[('c', 1), ('b', 2), ('a', 3)]
>>> d = {}
>>> d['c'] = 1
>>> d['b'] = 2
>>> d['a'] = 3
>>> d.items()
[('a', 3), ('c', 1), ('b', 2)]

So an OrderedDict does not order the elements for you, it preserves the order you give it.

If you want to "sort" a dictionary, you probably want

>>> sorted(d.items())
[('a', 1), ('b', 2), ('c', 3)]

回答2:

As of Python 3.7, a new improvement is:

the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec.

This means there is no real need for OrderedDict anymore