"Raymond Hettinger" <python@rcn.com> writes:

In Py2.3, __getitem__ conveniently supports slices for builtin sequences: 'abcde'.__getitem__(slice(2,4))

For user defined classes to emulate this behavior, they need to test the index argument to see whether it is a slice and then loop over the slice indices like this:

class SquaresToTen: """Acts like a list of squares but computes only when needed"""

def __len__(self): return 11

def __getitem__(self, index): if isinstance(index, slice): return [x**2 for x in range(index.start, index.stop, index.step)]

You can spell that if isinstance(index, slice): return [x**2 for x in range(*index.indices(11))] and, as a bonus, it'll work more often <wink> (consider "SquaresToTen()[7::-2]", for example).

This could be simplified somewhat by making slices iterable so that the __getitem__ definition looks more like this:

def __getitem__(self, index): if isinstance(index, slice): return [x**2 for x in index] else: return index**2

However to make omitted slice places work, you need to pass in the length of the sequence, so I don't think this can fly. Cheers, M. -- We've had a lot of problems going from glibc 2.0 to glibc 2.1. People claim binary compatibility. Except for functions they don't like. -- Peter Van Eynde, comp.lang.lisp