This supports my point that this ought to be handled once, correctly, in
the standard library, like the other ABCs, instead of leaving it up to people like me to get it wrong.
It would be pretty reasonable to assume that most users would get this incorrectly. My first assumption to this problem would've been something like ``hasattr(T, "__getitem__")``, but based on the current comments, it seems like that would be inadequate for this scenario. I'm not aware of any solution that would be remotely intuitive to users.
(2) If not, is there any reason why we shouldn't add Subscriptable
to the collection.abc module? I think I have the implementation:
Another important question might be "would users utilize the addition of Subscriptable in collections.abc enough for it to justify its addition to the standard library?". I think it looks interesting, but I think this should be implemented as a PyPI package (if something similar hasn't been already) so that we can evaluate it's usage rate before implementing it.
Raymond also brought up a strong point to consider in https://bugs.python.org/issue25988:
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.
On Fri, Sep 27, 2019 at 7:21 PM Steven D'Aprano firstname.lastname@example.org wrote:
On Fri, Sep 27, 2019 at 12:03:52PM -0700, Andrew Barnert via Python-ideas wrote:
On Sep 27, 2019, at 10:15, MRAB email@example.com wrote:
On 2019-09-27 17:05, Steven D'Aprano wrote:
But doing it correctly is too painful: if any(getattr(T, '__getitem__', None) is not None for T in
Is there are reason why you're using 'getattr' instead of 'hasattr'?
There is a difference here: if some class defines __getitem__ = None, blocks it from looking like a subscriptable type.
I wish I had been clever enough to have thought of that, but I didn't. I used getattr because I completely forgot about the existence of hasattr.
And I walked the MRO because oops.
On the other hand, the collections.abc walks the MRO explicitly checking for None, so perhaps I might have accidentally been less wrong than had I just tried hasattr(type(obj)). See the _check_methods function here:
This supports my point that this ought to be handled once, correctly, in the standard library, like the other ABCs, instead of leaving it up to people like me to get it wrong.
That idiom is used for other things like __hash__, even if it’s not used 100% consistently for all protocols.
If you look at the 3.7 collections.abc, _check_methods is now used consistently by all of the __subclasshook__ methods.
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