python - Difference between dictionary and OrderedDict

Question

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)

Answer 1

As of Python 3.7, a new improvement to the dict built-in 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 ??. They are almost the same.

---

Some minor details to consider...

Here are some comparisons between Python 3.7+ dict and OrderedDict:

from collections import OrderedDict

d = {'b': 1, 'a': 2}
od = OrderedDict([('b', 1), ('a', 2)])

# they are equal with content and order
assert d == od
assert list(d.items()) == list(od.items())
assert repr(dict(od)) == repr(d)

Obviously, there is a difference between the string representation of the two object, with the dict object in more natural and compact form.

str(d)  # {'b': 1, 'a': 2}
str(od) # OrderedDict([('b', 1), ('a', 2)])

As for different methods between the two, this question can be answered with set theory:

d_set = set(dir(d))
od_set = set(dir(od))
od_set.difference(d_set)
# {'__dict__', '__reversed__', 'move_to_end'}  for Python 3.7
# {'__dict__', 'move_to_end'}  for Python 3.8+

This means OrderedDict has at most two features that dict does not have built-in, but work-arounds are shown here:

Workaround for __reversed__ / reversed()

No workaround is really needed for Python 3.8+, which fixed this issue. OrderedDict can be "reversed", which simply reverses the keys (not the whole dictionary):

reversed(od)        # <odict_iterator at 0x7fc03f119888>
list(reversed(od))  # ['a', 'b']

# with Python 3.7:
reversed(d)                    # TypeError: 'dict' object is not reversible
list(reversed(list(d.keys()))) # ['a', 'b']

# with Python 3.8+:
reversed(d)        # <dict_reversekeyiterator at 0x16caf9d2a90>
list(reversed(d))  # ['a', 'b']

To properly reverse a whole dictionary using Python 3.7+:

dict(reversed(list(d.items())))  # {'a': 2, 'b': 1}

Workaround for move_to_end

OrderedDict has a move_to_end method, which is simple to implement:

od.move_to_end('b')  # now it is: OrderedDict([('a', 2), ('b', 1)])

d['b'] = d.pop('b')  # now it is: {'a': 2, 'b': 1}