[Python-Dev] [issue34221] Any plans to combine collections.OrderedDict with dict

Terry Reedy tjreedy at udel.edu
Thu Jul 26 13:23:06 EDT 2018

On 7/26/2018 2:15 AM, Raymond Hettinger wrote:

>On Jul 25, 2018, at 8:23 PM, INADA Naoki <songofacandy at gmail.com> wrote:

>> On Thu, Jul 26, 2018 at 12:04 PM Zhao Lee <redstone-cold at 163.com> wrote:
>>> Since Python 3.7,dicts remember the order that items were inserted, so any plans to combine collections.OrderedDict with dict?
>>> https://docs.python.org/3/library/collections.html?#collections.OrderedDict
>>> https://docs.python.org/3/library/stdtypes.html#dict

>> No.  There are some major difference.
>> * d1 == d2 ignores order / od1 == od2 compares order
>> * OrderedDict has move_to_end() method.
>> * OrderedDict.pop() takes `last=True` keyword.

> In addition to the API differences noted by Naoki, there are also implementation differences.  The regular dict implements a low-cost solution for common cases.  The OrderedDict has a more complex scheme that can handle frequent rearrangements (move_to_end operations) without touching, resizing, or reordering the underlying dictionary. Roughly speaking, regular dicts emphasize fast, space-efficient core dictionary operations over ordering requirements while OrderedDicts prioritize ordering operations over other considerations.
> That said, now that regular dicts are ordered by default, the need for collections.OrderedDict() should diminish quite a bit.  Mostly, I think people will ignore OrderedDict unless their application heavily exercises move to end operations.

On python-idea,  Miro Hrončok asked today whether we can change the 
OrderedDict repr from, for instance,

OrderedDict([('a', '1'), ('b', '2')]) # to
OrderedDict({'a': '1', 'b': '2'})

I am not sure what our repr change policy is, as there is a 
back-compatibility issue but I remember there being changes.

Terry Jan Reedy

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