I keep finding myself needing to test for objects that support
subscripting. This is one case where EAFP is *not* actually easier:
try:
obj[0]
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?
--
Steven
_______________________________________________
Python-ideas mailing list -- python-ideas@python.org
To unsubscribe send an email to python-ideas-le...@python.org
https://mail.python.org/mailman3/lists/python-ideas.python.org/
Message archived at
https://mail.python.org/archives/list/python-ideas@python.org/message/JCQJP5K32SELHRZFQB75CNLTECT3YD3K/
Code of Conduct: http://python.org/psf/codeofconduct/
