[issue35190] collections.abc.Sequence cannot be used to test whether a class provides a particular interface

[Miro Hrončok]

New submission from Miro Hrončok <miro at hroncok.cz>:

The collections.abc — Abstract Base Classes for Containers documentation says:

> This module provides abstract base classes that can be used to test whether a class provides a particular interface; for example, whether it is hashable or whether it is a mapping.

https://docs.python.org/3/library/collections.abc.html

However this is not true for Sequence.

When I implement a class that provides a particular interface (defined in the Collections Abstract Base Classes table in that very page), I cannot check whether it implements a Sequence.

See an example:

    from collections import abc

    class Box:
        def __init__(self, wrapped):
            self._w = wrapped

        def __len__(self):
            return len(self._w)

        def __iter__(self):
            yield from self._w

        def __getitem__(self, i):
            return self._w[i]

        def __reversed__(self):
            yield from reversed(self._w)

        def __contains__(self, i):
            return i in self._w

        def index(self, value, start=0, stop=None):
            return self._w.index(value, start, stop)

        def count(self, value):
            return self._w.count(value)


    b = Box([1, 2, 3])

    for t in 'Sized', 'Iterable', 'Reversible', 'Container', 'Collection', 'Sequence':
        print(f'{t}: {isinstance(b, getattr(abc, t))}')


My class is Reversible.
My class is a Collection (as it is a Sized Iterable Container).
It implements __getitem__, __len__, __contains__, __iter__, __reversed__, index, and count.

Yet my class instance is not an instance of Sequence.

I suppose this behavior might be intentional, as discussed in issue16728 - or it might as well not be.

The main concern was that dict also provides these methods, but is not considered a Sequence,
however dict does not provide index() or count().

Regardless whether this is right or wrong behavior, as documented this should be a Sequence.

See also https://stackoverflow.com/questions/34927949/issubclass-of-abstract-base-class-sequence


As I see it, either:

    collections.abc.Sequence needs a __subclasshook__ so it can be used as the documentation implies.

Or:

    the documentation should not say that "abstract base classes (from abc module) can be used to test whether a class provides a particular interface" if it doesn't generally apply


Or:

    the Sequence documentation should say: "this particular abstract base class cannot be used to test whether a class provides a particular interface because reasons" (yet I don't really get those reasons)

----------
assignee: docs at python
components: Documentation, Library (Lib)
messages: 329475
nosy: docs at python, hroncok, vstinner
priority: normal
severity: normal
status: open
title: collections.abc.Sequence cannot be used to test whether a class provides a particular interface
versions: Python 3.6, Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35190>
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