Unfortunate exception on dict item assignment (and why aren't slices hashable?)

[Bengt Richter]

On Sun, 20 Jul 2003 20:48:16 -0500, Jeff Epler <jepler at unpythonic.net> wrote:

>>>> {}[:] = None
>Traceback (most recent call last):
>  File "<stdin>", line 1, in ?
>TypeError: unhashable type
>
>I'd have expected something like 'TypeError: unslicable type'
>meaning that you can't slice a dict, not that slice()s aren't hashable.
>
>In fact, I wonder why slices *aren't* hashable.  I can imagine wanting
>to write something like the following:
Seems like you could (untested!)
>    class X:
         from types import SliceType  # BTW, why is slice a  builtin function, not a type?
>        # ...
>        def __getitem__(self, idx):
             if isinstance(idx, self.SliceType): idx = (idx.start, idx.stop, idx.step)
             # and adjust self.calculate_xxx to match.
>            if not idx in self._cache:
>                # potentially expire an item and
>                if isinstance(idx, slice):
>                    self._cache[idx] = self.calculate_slice_expensively(idx)
>                else:
>                    self._cache[idx] = self.calculate_idx_expensively(idx)
>            return self._cache[idx]
> 
>Jeff
>

Regards,
Bengt Richter