[Numpy-discussion] Allow __getitem__ to support custom objects

For ndindex (https://quansight.github.io/ndindex/), the biggest issue with the API is that to use an ndindex object to actually index an array, you have to use a[idx.raw] instead of a[idx]. This is because for NumPy arrays, you cannot allow custom objects to be indices. The exception is objects that define __index__, but this only works for integer indices. If __index__ returns anything other than an integer, you get an IndexError. This is annoying because it's easy to forget to do this when working with the ndindex API, and the error message from NumPy isn't informative about what went wrong unless you know to expect it. I'd like to propose an API that would allow custom objects to define how they should be converted to a standard NumPy index, similar to __index__ but that supports all index types. I think there are two options here: - Allow __index__ to return any index type, not just integers. This is the simplest because it reuses an existing API, and __index__ is the best possible name for this API. However, I'm not sure, but this may actually conflict with the text of PEP 357 (https://www.python.org/dev/peps/pep-0357/). Also, some other APIs use __index__ to check if something is an indexable integer, which wouldn't accept generic index. For example, elements of a slice can be any object that defines __index__. - Add a new __numpy_index__ API that works like def __numpy_index__(self): return In NumPy, __getitem__ and __setitem__ on ndarray would first check if the input index type is one of the known types as it currently does, then it would try __index__, and if neither of those fails, it would call __numpy_index__(index) and use that. Note: there is a more general way that NumPy arrays could allow __getitem__ to be defined on custom objects, which I am NOT proposing. Instead of an API that returns one of the current predefined index types (tuple, integer, slice, newaxis, ellipsis, or integer or boolean array), there could instead be an API that takes the array as input and returns another array (or view) as an output. This would allow an object to define itself as an index in arbitrary ways, even if such an index would not actually be possible via traditional indexing. There are definitely some interesting ideas that could be done with this, but this idea would be much more complicated, and isn't something that I need. Unless the community feels that a more general API like this would be preferred, I would suggest deferring something like it to a later discussion. What would be the best way to go about getting something like this implemented? Is it simple enough that we can just work out the details here and on a pull request, or should I write a NEP? Aaron Meurer