[Python-Dev] Add reversed methods for dict (original) (raw)

INADA Naoki songofacandy at gmail.com
Sun May 27 03:12:27 EDT 2018

On Sun, May 27, 2018 at 12:43 PM Raymond Hettinger < raymond.hettinger at gmail.com> wrote:

> On May 26, 2018, at 7:20 AM, INADA Naoki <songofacandy at gmail.com> wrote: > > Because doubly linked list is very memory inefficient, every implementation > would be forced to implement dict like PyPy (and CPython) for efficiency. > But I don't know much about current MicroPython and other Python > implementation's > plan to catch Python 3.6 up.

FWIW, Python 3.7 is the first Python that where the language guarantees that regular dicts are order preserving. And the feature being discussed in this thread is for Python 3.8.

Oh, my mistake.

What potential implementation obstacles do you foresee? Can you imagine any possible way that an implementation would have an order preserving dict but would be unable to trivially implement reversed? How could an implementation have a setitem that appends at the end, and a popitem() that pops from that same end, but still not be able to easily iterate in reverse? It really doesn't matter whether an implementer uses a dense array of keys or a doubly-linked-list; either way, looping backward is as easy as going forward.

I thought popitem() removes the last item is still implementation detail. So I thought about hashmap + single linked list. When removing item, dummy entry will be kept in the list. The dummy entry in the list will be removed when iterating over the list, or rebuilding hashmap.

FWIW, quick survey of other languages hashmap implementations and APIs are:

PHP

PHP 5 used hashmap + doubly linked list. PHP 7 uses Python-like implementation.

While PHP doesn't have reverse iterator, there are end() and prev() which can be used to iterate backwards.

Ruby

From Ruby 1.9, Hash is ordered. At the time, implementation is hashmap + doubly linked list. From Ruby 2.4, Python-like implementation.

There are Enumereble.reverse_each API. But the API is documented as "Builds a temporary array and traverses that array in reverse order." So Ruby seems allow other implementation which doesn't have zerocopy reverse iterator. (I don't know CRuby provides it or not.)

http://ruby-doc.org/core-2.2.2/Enumerable.html#method-i-reverse_each

Java

The LinkedHashMap document says " it maintains a doubly-linked list ". https://docs.oracle.com/javase/8/docs/api/java/util/LinkedHashMap.html

On the other hand, there are no reverse iterator API. So if we require __reverse__ for dict, Jython can't use LinkedHashMap as backend of dict.

C# (.Net)

There are legacy (non generic) OrderedDict. It's remove() seems O(n) implementation. https://referencesource.microsoft.com/#System/compmod/system/collections/specialized/ordereddictionary.cs,bc8d8035ee2d2927

Rust, Swift, and Go

Builtin mapping is arbitrary ordered, and there is no ordered mapping in the standard library.

It seems:

I may be wrong because I'm not expert of these languages. Please point out if I am wrong.

Raymond

P.S. It isn't going to be hard to update MicroPython to have a compact and ordered dict (based on my review of their existing dict implementation). This is something they are really going to want because of the improved memory efficiency. Also, they're also already going to need it just to comply with guaranteed keyword argument ordering and guaranteed ordering of class dictionaries.

Thanks.

Sadly speaking, Jython and IronPython development seems very slow and "wait until 3.9" may be not long enough for they catch Python 3.7 up.

When focusing to CPython, PyPy and MicroPython, no problem for adding reverse in 3.8 seems OK.

Regards,

-- INADA Naoki <songofacandy at gmail.com>

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