I keep finding myself needing to test for objects that support subscripting. This is one case where EAFP is *not* actually easier:
try: obj except TypeError: subscriptable = False except (IndexError, KeyError): subscriptable = True else: 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__ = () @abstractmethod def __getitem__(self, idx): return None @classmethod def __subclasshook__(cls, C): if cls is Subscriptable: return _check_methods(C, "__getitem__") return NotImplemented
Comments, questions, flames?