sd44 wrote:
> In <learning python>
> part III
> how does Python handle it if you try to extract a sequence in reverse,
> with the lower bound greater than the higher bound (e.g., L[3:1])? Hint:
> try
> assigning to this slice (L[3:1]=['?']), and see where the value is put.
>
>
>>>> L=[1,2,3,4]
>
>>>> L[3:1]
> []
>>>> print L
> [1, 2, 3, 4]
>>>> L[3:1]=['?']
>>>> print L
> [1, 2, 3, '?', 4]
To specify a backwards slice, you have to supply the third slice argument:
L[3:1:-1]
As it stands, `L[3:1]=['?']` replaces the empty sequence at index 3 with the
contents of ['?']. You know that `L[3:1]` is empty from the display above
the print statement.
The language manual has slicing defined as "a[i:j] selects all items with
ndex k such that i <= k < j". This is true as far as it goes but the
tutorial is truly on to something when it says "see where the value is put."
Python won't build a sequence that's less-than-empty, so it deals with an
empty sequence here. We're probably due for an argument on whether it
should raise an exception instead. See the current thread on whether `+`
should be a sequence concatenation operator.
Mel.
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