[issue25988] collections.abc.Indexable

[STINNER Victor]

STINNER Victor  added the comment:

New changeset c47c78b878ff617164b2b94ff711a6103e781753 by Hugo van Kemenade in 
branch 'master':
bpo-37324: Remove ABC aliases from collections (GH-23754)
https://github.com/python/cpython/commit/c47c78b878ff617164b2b94ff711a6103e781753

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[issue25988] collections.abc.Indexable

[STINNER Victor]

STINNER Victor  added the comment:


New changeset af5ee3ff610377ef446c2d88bbfcbb3dffaaf0c9 by Victor Stinner in 
branch 'master':
bpo-39674: Revert "bpo-25988: Do not expose abstract collection classes in the 
collections module. (GH-10596)" (GH-18545)
https://github.com/python/cpython/commit/af5ee3ff610377ef446c2d88bbfcbb3dffaaf0c9


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[issue25988] collections.abc.Indexable

[STINNER Victor]

Change by STINNER Victor :


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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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status: open -> closed
versions: +Python 3.9 -Python 3.7

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset ef092fe9905f61ca27889092ca1248a11aa74498 by Serhiy Storchaka in 
branch 'master':
bpo-25988: Do not expose abstract collection classes in the collections module. 
(GH-10596)
https://github.com/python/cpython/commit/ef092fe9905f61ca27889092ca1248a11aa74498


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[issue25988] collections.abc.Indexable

[Matthias Bussonnier]

Matthias Bussonnier  added the comment:

mbussonn: Your new PR looks like it's related to #36953 ("Remove collections 
ABCs?"), not this issue specifically. Can you withdraw/reissue attached to the 
correct issue?

Apologies, I've rebased and updated the issue numbers, not sure how I got the 
wrong one. I've also unlinked from here.

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[issue25988] collections.abc.Indexable

[Matthias Bussonnier]

Change by Matthias Bussonnier :


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[issue25988] collections.abc.Indexable

[Josh Rosenberg]

Josh Rosenberg  added the comment:

mbussonn: Your new PR looks like it's related to #36953 ("Remove collections 
ABCs?"), not this issue specifically. Can you withdraw/reissue attached to the 
correct issue?

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[issue25988] collections.abc.Indexable

[Matthias Bussonnier]

Change by Matthias Bussonnier :


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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Change by Guido van Rossum :


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[issue25988] collections.abc.Indexable

[Carl Bordum Hansen]

Carl Bordum Hansen  added the comment:

Thank you for all the hours all of you spent on making Python awesome.

I would like to start contributing as well.

Do you consider it time to remove `__getattr__` in 
`Lib/collections/__init__.py` on master yet (introduced in pr #5460)?

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[issue25988] collections.abc.Indexable

[Ned Deily]

Ned Deily  added the comment:


New changeset 2e84e47626c6eafacc9f011cd9fccc8bf1c8508e by Ned Deily in branch 
'3.7':
bpo-25988: add NEWS entry for 3.7.0b2 (#5743)
https://github.com/python/cpython/commit/2e84e47626c6eafacc9f011cd9fccc8bf1c8508e


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[issue25988] collections.abc.Indexable

[Ned Deily]

Change by Ned Deily :


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[issue25988] collections.abc.Indexable

[Ned Deily]

Ned Deily  added the comment:

Thanks, Tim, for noticing that and thanks, Ivan, for taking care of it.  I 
should have cherrypicked this into 3.7.0b1.

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[issue25988] collections.abc.Indexable

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:


New changeset 0442de5ad7835814d60f46c22a22942abb101aef by Ivan Levkivskyi in 
branch '3.7':
bpo-25988: Emit a warning when use or import ABCs from 'collections'. (GH-5734)
https://github.com/python/cpython/commit/0442de5ad7835814d60f46c22a22942abb101aef


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[issue25988] collections.abc.Indexable

[Ivan Levkivskyi]

Change by Ivan Levkivskyi :


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[issue25988] collections.abc.Indexable

[Tim Graham]

Tim Graham  added the comment:

The last commit that added the deprecation warning needs to be added to the 3.7 
branch.

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset c66f9f8d3909f588c251957d499599a1680e2320 by Serhiy Storchaka in 
branch 'master':
bpo-25988: Emit a warning when use or import ABCs from 'collections'. (#5460)
https://github.com/python/cpython/commit/c66f9f8d3909f588c251957d499599a1680e2320


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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum  added the comment:

Good idea, Victor, and thanks for the PR, Serhiy!

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

> Can't we use the new shiny PEP 562 here to actually emit a warning when 
> collections.Iterable is used instead of collections.abc.Iterable?

PR 5460 implements this.

It exposes issues in third-party packages. urllib3 and pyparsing directly use 
or import ABCs from 'collections'. request and pip are indirectly affected. If 
not add these warnings in 3.7 we can break the world in 3.8.

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue25988] collections.abc.Indexable

[STINNER Victor]

STINNER Victor  added the comment:

Can't we use the new shiny PEP 562 here to actually emit a warning when 
collections.Iterable is used instead of collections.abc.Iterable?

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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum  added the comment:

> > But we should make some noise around this change in the 3.7 announcement.
>
> How about a paragraph in the Deprecation section of the "What's New in Python 
> 3.7"? document
>
> https://docs.python.org/dev/whatsnew/3.7.html#deprecated

You mean other than what's there? The text that's currently there begins with 
"In Python 3.8, ..." which almost sounds like it's misplaced or a typo.  Maybe 
you can amend it somehow to explain that in 3.7, using or importing the ABCs 
from "collections" instead of from "collections.abc" is deprecated, and in 3.8 
it will stop working.

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[issue25988] collections.abc.Indexable

[Ned Deily]

Ned Deily  added the comment:

Guido:
> But we should make some noise around this change in the 3.7 announcement.

How about a paragraph in the Deprecation section of the "What's New in Python 
3.7"? document

https://docs.python.org/dev/whatsnew/3.7.html#deprecated

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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset e6d342156d2ab20fb88c0a5ec615fa8f602c0769 by Raymond Hettinger in 
branch 'master':
bpo-25988: Deprecate exposing collections.abc in collections GH-5414
https://github.com/python/cpython/commit/e6d342156d2ab20fb88c0a5ec615fa8f602c0769


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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Change by Raymond Hettinger :


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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum added the comment:

I think it's not worth the hack that Serhiy suggests. But we should make
some noise around this change in the 3.7 announcement.

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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Raymond Hettinger added the comment:

> The copying approach likely won't work ...

Then it seems like just adding a note to the docs is the way to go. That's 
probably okay though.  I don't think we've every gotten much benefit from the 
programmatic deprecation warnings.

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

If we want to add programming warnings, we should replace 
sys.modules['collections'] with an instance of a module type subclass with 
corresponding properties raising a DeprecationWarning.

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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum added the comment:

The copying approach likely won't work because there will be code that uses 
isinstance(x, collections.abc.Sequence) and other code that uses isinstance(x, 
collections.Sequence) and they'll expect both to be true.

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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Raymond Hettinger added the comment:

> Yes we will need a deprecation period for this starting with 3.7.

There are two ways to do this.  A wimpy and clean way is to just add a 
deprecation notice in the docs.  The thorough and messy way is to temporarily 
copy everything from the 3.6 collections.abc into the regular collections 
module and then insert deprecation warning code directly in each of copied 
classes.

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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum added the comment:

Yes we will need a deprecation period for this starting with 3.7. It's fine
not to import names newly added to collections.abc in 3.7 (i.e. collections
would have to explicitly import everything that it got via * in 3.6).

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[issue25988] collections.abc.Indexable

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I'm not sure that *new* names in collections.abc should be re-exported to 
collections. This isn't required for backward compatibility.

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[issue25988] collections.abc.Indexable

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Names from collections.abc are re-exported to collections for backward 
compatibility. IIRC Serhiy also wanted to stop re-exporting them. I am not sure 
whether we need any "deprecation period" for this.

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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Raymond Hettinger added the comment:

Guido, do you think there is some way to decouple collections.abc from spilling 
into the namespace for non-abc collections module?

At the interactive prompt, running dir() on collections gives an alphabetical 
hodgepodge of the two modules.  To my eyes, it is difficult to discern the 
concrete from the abstract.

>>> dir(collections)
['AsyncGenerator', 'AsyncIterable', 'AsyncIterator', 'Awaitable', 'ByteString', 
'Callable', 'ChainMap', 'Collection', 'Container', 'Coroutine', 'Counter', 
'Generator', 'Hashable', 'ItemsView', 'Iterable', 'Iterator', 'KeysView', 
'Mapping', 'MappingView', 'MutableMapping', 'MutableSequence', 'MutableSet', 
'OrderedDict', 'Reversible', 'Sequence', 'Set', 'Sized', 'UserDict', 
'UserList', 'UserString', 'ValuesView', '_Link', '_OrderedDictItemsView', 
'_OrderedDictKeysView', '_OrderedDictValuesView', '__all__', '__builtins__', 
'__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', 
'__path__', '__spec__', '_chain', '_class_template', '_collections_abc', 
'_count_elements', '_eq', '_field_template', '_heapq', '_iskeyword', 
'_itemgetter', '_proxy', '_recursive_repr', '_repeat', '_repr_template', 
'_starmap', '_sys', 'abc', 'defaultdict', 'deque', 'namedtuple']

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[issue25988] collections.abc.Indexable

[Guido van Rossum]

Guido van Rossum added the comment:

At least one use of Indexable (by whatever name, but this rolls off the tongue 
the easiest) would be the metaclass in typing.py that allows one to write 
List[int] -- there's no containter here!

The "drowning out" seems purely subjective -- perhaps if you order the contents 
of the module alphabetically you won't get much insight, but the docs should 
use a better organizing principle than that. (After all alphabetization was 
invented as a crutch for searching. :-)

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[issue25988] collections.abc.Indexable

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Raymond,

> The existence of use ABCs like MutableMapping is being drowned-out by 
> one-trick-ponies.  We're developing an unfavorable ratio of theoretical 
> building blocks versus the practical tools.

Why do you think they are "theoretical"? FWIW, a simple search on GitHub for 
abc.X gives this:
Sequence 322K
Mapping 267K
Iterable 244K
Container 99K
MutableMapping 77K

I don't see any pattern here (although this may be only anecdotal evidence :-)

> Other than a feeling of lightness, I don't think this proposal does anything 
> for us.

This proposal makes more sense (and motivation) in the context of static 
typing, since it is a good way to know how __getitem__ is going to be used -- 
from its static type. Something typed with Subscriptable[int, T] would accept 
both Sequence[T] and Mapping[int, T]. The latter two are  currently 
differentiated by __reversed__ (Sequence is Reversible as opposed to Mapping).

Concerning an invariant that order of iteration is consistent with indexing by 
subsequent integers, I think this can't be checked reliably neither statically, 
nor by any simple runtime isinstance check. For example a subclass can override 
Indexable.__iter__ and break the iteration order. We can still add it, and rely 
on user cooperation.

Anyway, I am not insisting on adding either Subscriptable nor Indexable, but I 
think it is important that these things are discussed in view of PEP 544.

Alternative proposal would be to add Subscriptable and its subclass Indexable 
only to typing as protocols (with some special-casing in type checkers for the 
latter to provide a higher level of contract).
(Concerning the name, I think Enumerable sounds better than Indexable, IIUC the 
idea is to work correctly with enumerate.)

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[issue25988] collections.abc.Indexable

[Raymond Hettinger]

Raymond Hettinger added the comment:

IIRC, the original motivation for ABCs was to differentiate distinct uses of 
__getitem__ (we forever struggled with differentiating sequences from mapping). 
 It seems to me that this proposal is a step backwards.  Other than a feeling 
of lightness, I don't think this proposal does anything for us.  What is point 
of knowing an object is Subscriptable without knowing how it is to be used.

The OP has a sense that Mapping and Sequence are "too heavy" but I think the 
reality that useful classes almost never use __getitem__ in isolation; rather, 
it is part of a small constellation of methods that are typically used 
together.  I would prefer that collections.abc continue to reflect that reality.

Also, I worry that collections.abc is becoming cluttered.  The existence of use 
ABCs like MutableMapping is being drowned-out by one-trick-ponies.  We're 
developing an unfavorable ratio of theoretical building blocks versus the 
practical tools.

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[issue25988] collections.abc.Indexable

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

The last few weeks something bothered while working on Protocols PEP: protocols 
should be ideally compact (and PEP already emphasizes this).
However, the only potential candidates for __getitem__ are Sequence and 
Mapping, that are both quite bulky (half dozen members each). I was thinking 
about different options like adding BaseMap as a base for Mapping and Sequence 
that will only have __getitem__. I expect this to be a popular protocol, since 
often people just need something that can be subscripted.

Fortunately I stumbled into this issue. It looks like the optimal way now is:

* Have an abstract base class (let's call it BaseMap, although I don't really 
like this name) in collections.abc that has only __getitem__ method.
* It will be inherited by both Sequence and Mapping, but for the purpose of 
static typing, Sequence[T] will be a subtype of BaseMap[int, T], while 
Mapping[KT, VT] will be a subtype of BaseMap[KT, VT].
* BaseMap will be contravariant in key, this will solve problems with Mapping 
(invariant in key), for example a function that expects BaseMap[str, int] will 
accept Dict[Union[str, unicode], int].
* BaseMap will be a protocol in typing, so that people can extend it depending 
on their needs (e.g. add a .get() method).

Guido, Łukasz if you agree, then I will add this to the Protocols PEP and  will 
make a PR to collections.abc. We need to agree on the name, there are two 
options now: BaseMap and Indexable. I don't like the second since I would 
rather have it as a generic alias: Indexable = BaseMap[int, T]. However, 
BaseMap is also not very good, since I want something more "neutral" between 
Mapping and Sequence.

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nosy: +levkivskyi, lukasz.langa
versions: +Python 3.7

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[issue25988] collections.abc.Indexable

[Andrew Barnert]

New submission from Andrew Barnert:

In an -ideas thread, Guido suggested 
(http://article.gmane.org/gmane.comp.python.ideas/37599):

> If we want some way to turn something that just defines __getitem__ and 
> __len__ into a proper sequence, it should just be made to inherit from 
> Sequence, which supplies the default __iter__ and __reversed__. (Registration 
> is *not* good enough here.) If we really want a way to turn something that 
> just supports __getitem__ into an Iterable maybe we can provide an additional 
> ABC for that purpose; let's call it a HalfSequence until we've come up with a 
> better name. (We can't use Iterable for this because Iterable should not 
> reference __getitem__.)

Later in the thread, Nick Coghlan suggested 
(http://article.gmane.org/gmane.comp.python.ideas/37614):

> Perhaps collections.abc.Indexable would work? Invariant:

> for idx, val in enumerate(container):
> assert container[idx] is val

> That is, while enumerate() accepts any iterable, Indexable containers
have the additional property that the contained values can be looked
up by their enumeration index. Mappings (even ordered ones) don't
qualify, since they offer a key:value lookup, but enumerating them
produces an index:key relationship.

So, in particular:

* Indexable is a subclass of Iterable.
* Sequence is a subclass of Indexable, Sized, and Container instead of 
Iterable, Sized, and Container. (Or, if #25987 also goes through, of 
Reversible, Indexable, Sized, and Container.)
* The abstract method __getitem__ and the concrete __iter__ implementation get 
moved up from Sequence to Indexable.
* Indexable does _not_ have a subclass hook (to avoid making every Mapping, 
generic type, etc. accidentally Indexable).

This means that you can write this (borrowing an example from Steven D'Aprano 
in http://article.gmane.org/gmane.comp.python.ideas/23369/):

class Squares(collections.abc.Indexable):
def __getitem__(self, index):
return index**2

Because this no longer depends on the old-style sequence protocol, testing it 
with ABCs will work as expected.

For related issues, see #25987, #25864, #25958, and 
https://github.com/ambv/typehinting/issues/170

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components: Library (Lib)
messages: 257311
nosy: abarnert
priority: normal
severity: normal
status: open
title: collections.abc.Indexable
type: enhancement

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