Just FYI there is a closely related issue on b.p.o. https://bugs.python.org/issue25988. FWIW I am in favor of the idea, but some people are against it (see e.g. the issue).


On Fri, 27 Sep 2019 at 17:12, Steven D'Aprano <steve@pearwood.info> wrote:
I keep finding myself needing to test for objects that support
subscripting. This is one case where EAFP is *not* actually easier:

    except TypeError:
        subscriptable = False
    except (IndexError, KeyError):
        subscriptable = True
        subscriptable = True
    if subscriptable:

But I don't like manually testing for it like this:

    if getattr(obj, '__getitem__', None) is not None: ...

because it is wrong. (Its wrong because an object with __getitem__
defined as an instance attribute isn't subscriptable; it has to be in
the class, or a superclass.)

But doing it correctly is too painful:

    if any(getattr(T, '__getitem__', None) is not None for T in type(obj).mro())

and besides I've probably still got it wrong in some subtle way. What
I'd really like to do is use the collections.abc module to do the check:

    if isinstance(obj, collections.abc.Subscriptable): ...

in the same way we can check for Sized, Hashable etc.

Alternatively, if we had a getclassattr that skipped the instance
attributes, I could say:

    if getclassattr(obj, '__getitem__', None) is not None: ...

(1) Am I doing it wrong? Perhaps I've missed some already existing
solution to this.

(2) If not, is there any reason why we shouldn't add Subscriptable to
the collection.abc module? I think I have the implementation:

    class Subscriptable(metaclass=ABCMeta):
        __slots__ = ()
        def __getitem__(self, idx):
            return None
        def __subclasshook__(cls, C):
            if cls is Subscriptable:
                return _check_methods(C, "__getitem__")
            return NotImplemented

Comments, questions, flames?

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