On Fri, Apr 23, 2021 at 10:33 AM Chris Angelico <rosuav@gmail.com> wrote:
On Fri, Apr 23, 2021 at 11:22 AM Larry Hastings <larry@hastings.org> wrote:
On 4/20/21 10:03 AM, Mark Shannon wrote:
If you guarded your code with `isinstance(foo, Sequence)` then I could not use it with my `Foo` even if my `Foo` quacked like a sequence. I was forced to use nominal typing; inheriting from Sequence, or explicitly registering as a Sequence.
If I'm reading the library correctly, this is correct--but, perhaps, it could be remedied by adding a __subclasshook__ to Sequence that looked for an __iter__ attribute. That technique might also apply to other ABCs in collections.abc, Mapping for example. Would that work, or am I missing an critical detail?
How would you distinguish between a Sequence and a Mapping? Both have __iter__ and __len__. Without actually calling those methods, how would the subclass hook tell them apart?
ChrisA
We can add .keys() to Mapping to distinguish Mapping and Sequence. But it is breaking change, of course. We shouldn't change it. I think using ABC to distinguish sequence or mapping is a bad idea. There are three policies: a) Use duck-typing; just us it as sequence. No type check at all. b) Use strict type checking; isinstance(x, list) / isinstance(x, (list, tuple)). c) Use ABC. But (c) is broken by design. It is not fixable. IMHO, We should chose (a) or (b) and reject any idea relying on Sequence ABC. Regards, -- Inada Naoki <songofacandy@gmail.com>