[issue41409] deque.pop(index) is not supported

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

I am convinced by Raymond's argument, it seems that with the current state of 
the ABC classes and semantics the most pragmatical solution is the status quo. 
It would be a weird if deque is a Sequence but not a MutableSequence because it 
can clearly be mutated.

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[issue41409] deque.pop(index) is not supported

[Raymond Hettinger]

Raymond Hettinger  added the comment:

> If deque does not fully support the MutableSequence interface,
> should not it be un-regitered as MutableSequence?

You could unregister it, but for such a minor variance, it isn't worth it.  
We're not unregistering sets and frozenset from Set even though there are minor 
variances in the operators. 

The ABCs are quite limited in what they do. The underlying machinery is mostly 
about determining whether a method is present or not.  Concrete classes are 
free to override, extend or raise NotImplemented.  For example, a Counter is 
still a mapping even though fromkeys() raises NotImplemented and update() has 
different semantics than Mapping.  

Jelle has already adapted TypeShed to match the concrete class, so no actual 
user issue remains.

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[issue41409] deque.pop(index) is not supported

[Akuli]

Akuli  added the comment:

I don't think it's very common to write code that needs to work with any 
MutableSequence but not with any Sequence. I think that's the only situation 
where missing support for deque.pop(index) is a problem.

Maybe deque should be a Sequence but not a MutableSequence. Or maybe there 
should be a way to say "MutableSequence does not require support for 
.pop(index) even though you get it by inheriting from MutableSequence".

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[issue41409] deque.pop(index) is not supported

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

If deque does not fully support the MutableSequence interface, should not it be 
un-regitered as MutableSequence? Maybe we need other abstract class which would 
be parent of MutableSequence and deque?

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nosy: +stutzbach

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[issue41409] deque.pop(index) is not supported

[Raymond Hettinger]

Raymond Hettinger  added the comment:

FWIW, the relationship between a concrete class and an ABC is normative with 
respect to core capabilities but isn't 100% strict.  Concrete classes can vary 
in small respects when it makes sense.  For example, the __or__ and __and__ 
methods for collections.Set will accept any iterable, but the concrete does 
not.  

We use the Liskov substitution principle as a guide, not as a law.  In a number 
of cases, we choose practicality-beats-purity and allow subclasses to differ in 
ways than their parents.  Counter() doesn't support fromkeys().  
OrderedDict.popitem() has different signature than dict.popitem().

Hope that insight was helpful.

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[issue41409] deque.pop(index) is not supported

[Raymond Hettinger]

Raymond Hettinger  added the comment:

This was an intentional decision.  Deques designed for fast access at the end 
points. Also, pop() is a core deque operation that needs to be fast.  Altering 
its signature with an optional argument would slow it down.

Thank you for the suggestion, but we really shouldn't do this.

--
resolution:  -> rejected
stage:  -> resolved
status: open -> closed
versions: +Python 3.10

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[issue41409] deque.pop(index) is not supported

[Jim Jewett]

Jim Jewett  added the comment:

It may well have been intentional, as deques should normally be mutated only at 
the ends.  But Raymond did make changes to conform to the ABC, so this should 
probably be supported too.  Go ahead and include docstrings and/or discouraging 
it, though, except for i=0 and i=-1

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nosy: +Jim.Jewett

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[issue41409] deque.pop(index) is not supported

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> Hi, I want to start contributing to CPython. Can I take up this issue?

Sure, go ahead. Feel free to ping one of us in the PR for review

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nosy: +pablogsal

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[issue41409] deque.pop(index) is not supported

[Shubham Kumar Jha]

Shubham Kumar Jha  added the comment:

Hi, I want to start contributing to CPython. Can I take up this issue?

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nosy: +ShubhamKJha

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[issue41409] deque.pop(index) is not supported

[Yuan]

Yuan  added the comment:

Same status as slicing support from MutableSequence.

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nosy: +Yuan

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[issue41409] deque.pop(index) is not supported

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

>>> issubclass(collections.deque, collections.abc.MutableSequence)
True
>>> sorted(set(dir(collections.abc.MutableSequence)) - 
>>> set(dir(collections.deque)))
['__abstractmethods__', '__module__', '__slots__', '_abc_impl']

Well, it is a bug.

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type:  -> behavior

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[issue41409] deque.pop(index) is not supported

[Akuli]

Akuli  added the comment:

I meant MutableSequence instead of MutableMapping. Oops.

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[issue41409] deque.pop(index) is not supported

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

deque is not a subclass of MutableMapping, so the Liskov substitution principle 
is not related here.

deque is not registered as a virtual subclass of MutableMapping and it lacks a 
number of MutableMapping methods.

>>> import collections.abc
>>> issubclass(collections.deque, collections.abc.MutableMapping)
False
>>> sorted(set(dir(collections.abc.MutableMapping)) - 
>>> set(dir(collections.deque)))
['_MutableMapping__marker', '__abstractmethods__', '__module__', '__slots__', 
'_abc_impl', 'get', 'items', 'keys', 'popitem', 'setdefault', 'update', 
'values']

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nosy: +serhiy.storchaka

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[issue41409] deque.pop(index) is not supported

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


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nosy: +rhettinger

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[issue41409] deque.pop(index) is not supported

[Akuli]

New submission from Akuli :

The pop method of collections.deque can't be used like deque.pop(index), even 
though `del deque[index]` or deque.pop() without an argument works. This breaks 
the Liskov substitution principle because collections.abc.MutableMapping 
supports the .pop(index) usage. Is this intentional?

related typeshed issue: https://github.com/python/typeshed/issues/4364

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components: Library (Lib)
messages: 374378
nosy: Akuli
priority: normal
severity: normal
status: open
title: deque.pop(index) is not supported

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