Question

It’s not a real world program but I would like to know why it can’t be done.

I was thinking about numpy.r_ object and tried to do something similar but just making a class and not instantiating it.

a simple code (has some flaws) for integers could be:

class r_:
    @classmethod
    def __getitem__(clc, sl):
        try:
            return range(sl)
        except TypeError:
            sl = sl.start, sl.stop, sl.step
            return range(*(i for i in sl if i is not None))

but as I try to do r_[1:10] i receive TypeError: 'type' object is not subscriptable.

Of course the code works with r_.__getitem__(slice(1,10)) but that’s not what I want.

Is there something I can do in this case instead of using r_()[1:10]?

Answer 1

The protocol for resolving obj[index] is to look for a __getitem__ method in the type of obj, not to directly look up a method on obj (which would normally fall back to looking up a method on the type if obj didn’t have an instance attribute with the name __getitem__).

This can be easily verified.

>>> class Foo(object):
    pass

>>> def __getitem__(self, index):
    return index

>>> f = Foo()
>>> f.__getitem__ = __getitem__
>>> f[3]
Traceback (most recent call last):
  File "<pyshell#8>", line 1, in <module>
    f[3]
TypeError: 'Foo' object does not support indexing
>>> Foo.__getitem__ = __getitem__
>>> f[3]
3

I don’t know why exactly it works this way, but I would guess that at least part of the reason is exactly to prevent what you’re trying to do; it would be surprising if every class that defined __getitem__ so that its instances were indexable accidentally gained the ability to be indexed itself. In the overwhelming majority of cases, code that tries to index a class will be a bug, so if the __getitem__ method happened to be able to return something, it would be bad if that didn’t get caught.

Why don’t you just call the class something else, and bind an instance of it to the name r_? Then you’d be able to do r_[1:10].