[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

Joshua Bronson added the comment:

For the record, it looks like Victor Stinner suggested doing this in 
https://mail.python.org/pipermail/python-dev/2016-September/146349.html

Brett Cannon replied in 
https://mail.python.org/pipermail/python-dev/2016-September/146350.html to 
suggest adding "ordered mapping" to the glossary instead. I took a quick look 
at https://docs.python.org/3.6/glossary.html and don't see it there.

--
nosy: +brett.cannon, haypo

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[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

Joshua Bronson added the comment:

Sorry to hear but thanks for the consideration. To follow up on your comments:

> nice to see Guido's reasons for giving a -0 on the proposal.

(Guido was giving his -0 on a proposal for collections.abc.Ordered, whereas the 
main proposal here is collections.abc.OrderedMapping, sorry if that was 
confusing.)


> I would add that I haven't see any code in the wild that would benefit from 
> testing isinstance(m, OrderedMapping) and taking some different action 
> depending on the result.

Just to give you another data point, bidict.orderedbidict has been released 
into the wild for a little over 10 months, and people are using it. 
OrderedMapping would give users the following benefits:

>>> noble = orderedbidict([(2, 'helium'), (10, 'neon'), (18, 'argon')])

>>> b = bidict(noble)  # Discards the order.
>>> b  # Calls BidictBase.__repr__.
bidict({10: 'neon', 18: 'argon', 2: 'helium'})

>>> noble  # Also calls BidictBase.__repr__.
orderedbidict([('Cairo', 'Egypt'), ('Lima', 'Peru'), ('Tokyo', 'Japan')])

>>> # i.e. BidictBase.__repr__ interrogates isinstance(self, OrderedMapping)
>>> # to reflect any ordering correctly in the string representation.

>>> # OrderedMapping.__eq__ also checks isinstance(other, OrderedMapping)
>>> # to conditionally turn on order-sensitivity:
>>> noble == collections.OrderedDict(noble.items())
True
>>> noble == collections.OrderedDict(reversed(noble.items()))
False
>>> noble == dict(reversed(noble.items()))  # order-insensitive
True

(collections.OrderedDict and sortedcontainers.SortedDict could reap these same 
benefits from an OrderedMapping class.)


> Grant's sortedcollection is more sequence-like than mapping-like and the 
> bidict has its own interesting properties and neither are substitutable for 
> an OrderedDict.

Hm, I don't quite follow this. OrderedDict, SortedDict, and orderedbidict are 
all OrderedMappings, and should therefore be passable to any function that 
expects an OrderedMapping.

This part I do follow:

> In other words, the only properties these cluster on is the ordered equality 
> feature.  IMO, that particular feature hasn't established itself as being 
> valuable It is an interesting idea but I believe it that it would clutter 
> the module, slightly drowning-out the ABCs that have more broad and 
> established utility.

But based on this rationale, I would have thought Reversible wouldn't have been 
added. It was only after I found https://bugs.python.org/issue25987 the other 
day and saw that Reversible will be in 3.6 that I thought OrderedMapping had a 
case of similar strength, and so it seemed worth taking the time to try to 
contribute it for 3.7.

In any case, thanks again for your consideration, and if you have any further 
thoughts to share, it'd be much appreciated.

--
nosy: +abarnert

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[issue28912] collections.abc.OrderedMapping

[Raymond Hettinger]

Raymond Hettinger added the comment:

Thank you for the link, it was nice to see Guido's reasons for giving a -0 on 
the proposal.  I would add that I haven't see any code in the wild that would 
benefit from testing isinstance(m, OrderedMapping) and taking some different 
action depending on the result.

Grant's sortedcollection is more sequence-like than mapping-like and the bidict 
has its own interesting properties and neither are substitutable for an 
OrderedDict.  In other words, the only properties these cluster on is the 
ordered equality feature.  IMO, that particular feature hasn't established 
itself as being valuable (we don't see much use of it with the existing ordered 
dicts, and likewise in the database world, table equivalence based on exact 
order rather than content equivalence is uncommon).

Given the limited utility, I am going to decline the proposal.  It is an 
interesting idea but I believe it that it would clutter the module, slightly 
drowning-out the ABCs that have more broad and established utility.

One other thought is that the proposed OrderedMapping ABC might not fit well in 
a possible future Python where the behavior Python 3.6 ordered-and-compact 
dicts becomes guaranteed but where we retain the traditional unordered equality 
comparison.

--
resolution:  -> rejected
status: open -> closed

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[issue28912] collections.abc.OrderedMapping

[Raymond Hettinger]

Changes by Raymond Hettinger :


--
assignee:  -> rhettinger

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[issue28912] collections.abc.OrderedMapping

[Guido van Rossum]

Changes by Guido van Rossum :


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nosy:  -gvanrossum

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[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

Joshua Bronson added the comment:

I only just found the "[Python-ideas] Adding collections.abc.Ordered" thread at 
https://mail.python.org/pipermail/python-ideas/2015-November/037146.html - 
sorry for not seeing it sooner. Looking forward to catching up on what I missed.

--

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[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

Joshua Bronson added the comment:

Come to think of it, to be exact, rather than extending Reversible, 
OrderedMapping could extend a narrower interface, something like 
collections.abc.Ordered, along with extending Mapping. (Reversible implies 
Ordered, but Ordered does not imply Reversible: a singly-linked list is Ordered 
but not Reversible, for example.)

I guess we don't currently have Ordered because it wouldn't necessarily add any 
abstractmethods beyond what Iterable already provides. But it could provide a 
generic, concrete __eq__ implementation, something like:

class Ordered(Iterable):
def __eq__(self, other):
if not isinstance(other, Ordered):
return NotImplemented
return all(i == j for (i, j) in zip(self, other))


which is not something that any existing collections.abc Iterable currently 
provides.

Even if an Ordered interface that's totally empty were available in the 
standard library, then Iterables across diparate codebases could opt into 
extending it as a standard signal that they iterate over their elements in a 
well-defined order, and could participate in other polymorphic code. Currently 
we have no such standard signal, do we?

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[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

Joshua Bronson added the comment:

This patch improves the OrderedMapping.__eq__ implementation to be more generic 
in the case that ``other`` is an unordered Mapping of the same length as 
``self``.

--
Added file: http://bugs.python.org/file45805/jab-orderedmapping-2.patch

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[issue28912] collections.abc.OrderedMapping

[Joshua Bronson]

New submission from Joshua Bronson:

Since code can be clearer than prose, I just sketched this idea out in the 
attached patch. Please take a look at it as a minimum demonstration of the 
concept.

Rationale:

The Python standard library provides collections.OrderedDict, along with 
several ABCs which OrderedDict extends, such as collections.abc.Mapping and (as 
of 3.6) collections.abc.Reversible, to enable better composability and 
interoperability.

Currently there is no collections.abc.OrderedMapping ABC, but I wonder if there 
should be. OrderedMapping could provide a concrete, generic implementation of 
__eq__, that performed an order-sensitive comparison when passed another 
OrderedMapping, while performing an order-insensitive comparison when passed an 
unordered Mapping. Then OrderedDict could derive from OrderedMapping, 
inheriting its generic __eq__ implementation, rather than implementing its own 
__eq__ method. Currently, OrderedDict's own __eq__ implementation does 
``isinstance(other, OrderedDict)`` to decide whether to perform an 
order-sensitive comparison, which thwarts interoperability. OrderedMapping 
could also derive from Reversible, signaling that classes that extend it 
implement __reversed__.

The interoperability gain here is not just theoretical. Several packages are 
published on PyPI which would be able to interoperate better if they could all 
extend a common OrderedMapping interface. Grant Jenks' 
sortedcontainers.SortedDict[1] and my own bidict.OrderedBidict[2] are two 
particularly popular examples.

Thanks for your consideration, and look forward to your feedback.

[1] http://www.grantjenks.com/docs/sortedcontainers/sorteddict.html
[2] https://bidict.readthedocs.io/en/dev/other-bidict-types.html#orderedbidict

--
components: Library (Lib)
files: jab-orderedmapping-1.patch
keywords: patch
messages: 282742
nosy: gvanrossum, jab, rhettinger
priority: normal
severity: normal
status: open
title: collections.abc.OrderedMapping
type: enhancement
Added file: http://bugs.python.org/file45804/jab-orderedmapping-1.patch

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