Issue 36455: collections.abc.Sequence doesn't support reflection/introspection (original) (raw)

Consider:

from collections.abc import *

class DefinitelyNotAList:
    def __init__(self, backer):
        self.backer = backer

    def __getitem__(self, key):
        return self.backer[key]

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

    def __contains__(self, needle):
        return needle in self.backer

    def __reversed__(self):
        return reversed(self.backer)

    def __iter__(self):
        return list.__iter__(self.backer)

    def index(self, *args, **kwargs): return self.backer.index(*args, **kwargs)

    def count(self, *args, **kwargs): return self.backer.count(*args, **kwargs)


dnal = DefinitelyNotAList([2,4,6,8])

for abc in [Collection, Reversible, Sized, Iterable, Container, Sequence]:
    print(abc.__name__.ljust(12), isinstance(dnal, abc))

Which prints:

Collection   True
Reversible   True
Sized        True
Iterable     True
Container    True
Sequence     False

I'm not sure whether this is a bug, a docs bug, or just a misunderstanding but my expectation, is that, somehow, I could get isinstance(obj, Sequence) to return without explicitly registering it, just as True is returned for the other abcs.

Edit conflict -- indeed -- self closing.

I should point out that this could be fixed with something like the following:

@classmethod
def __subclasshook__(cls, C):
    if cls is Sequence:
        return _check_methods(C, "__reversed__", "__iter__",
                              "__len__", "__contains__", "__getitem__")
    return NotImplemented

But seeing as it's not the only abc that is without a subclass hook, I wanted to raise an issue before I submitted a complete PR implementing subclasshooks for the rest of the abcs that need them.