[Python-Dev] non-integer slice indices?

[Christian Tismer]

Hi Skip,

Skip Montanaro wrote:
> 
>     Greg> Currently, a person can do the following:
> 
>     Greg>     foo[slice('a','abc')] = some_sequence
> 
> Well, I'll be damned!  To wit:

[nice implementation cut]

> Now, about that motivation...

usually I'm not the one to argue against a new feature, but
I think this extension to slicing is too much and not
consistent enough.

When we write
x[low:high] = some_sequence

then we imply that there is a sequence on the left hand that
can be indexed by the implicit ordered set of integers in
the range [low, high), and we allow this assignment to change
the sequence's length arbitrarily.

Speaking of mapping objects, you specify a set of values
by an expression of their keys, but you have no way to
invent new keys, only deletion applies. Appears a bit
twisted to do this to a mapping.

A different approach would be to require a mapping object
on the right hand. The assignment would have to
1) check that all keys on the right are inside the lice's range
2) delete the entries in that range from the left
3) insert the new keys/values.

Indexing a mapping by a slice should return a mapping again.

Well, I don't like any of these so much. They make dicts look
like something ordered, that rings a bell about too much
cheating.

Or we could be consequent and provide a sequence protocol
for mappings as well, with all that sort-on-demand consequences
necessary. But this is not possible since integers can be keys,
and it would be undecidable wether we want sequence indexing
or mapping indexing. This would only make sense for typed
dictionaries, which allow string keys only for instance.

I'd say better drop it - chris

-- 
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