Will OrderedDict become redundant in Python 3.7?

Question

From the Python 3.7 changelog:

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

Would this mean that OrderedDict will become redundant? The only use I can think of it will be to maintain backwards compatibility with older versions of Python which don't preserve insertion-order for normal dictionaries.

Solution

No it won't become redundant in Python 3.7 because OrderedDict is not just a dict that retains insertion order, it also offers an order dependent method, OrderedDict.move_to_end(), and supports reversed() iteration*.

Moreover, equality comparisons with OrderedDict are order sensitive and this is still not the case for dict in Python 3.7, for example:

>>> OrderedDict([(1,1), (2,2)]) == OrderedDict([(2,2), (1,1)]) 
False
>>> dict([(1,1), (2,2)]) == dict([(2,2), (1,1)]) 
True

Two relevant questions here and here.

* Support for reversed() iteration of regular Python dict is added for Python 3.8, see issue33462

Comments

[Boris Verkhovskiy]

See also: stackoverflow.com/questions/34305003/…

[Tim Skov Jacobsen]

The point about order sensitivity when comparing is very valid here.

[leggewie]

Thank you very much for the explanation. I believe that with 3.8, there is no such difference anymore.

[Chris_Rands]

@leggewie In Python 3.8 (and in 3.9), the point about equality comparisons is still different between OrderedDict and regular dict- I do not expect this to change anytime soon