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)]
else:
return index**2
print SquaresToTen()[2]
print SquaresToTen()[7:1:-2]
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
Raymond Hettinger
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