On 18/05/2015 22:04, Mario Figueiredo wrote:
On Mon, 18 May 2015 13:49:45 -0600, Ian Kelly <ian.g.ke...@gmail.com>
wrote:
Other languages implement slices. I'm currently being faced with a Go
snippet that mirrors the exact code above and it does run in linear
time.
Is there any reason why Python 3.4 implementation of slices cannot be
a near constant operation?
The semantics are different. IIRC, a slice in Go is just a view of
some underlying array; if you change the array (or some other slice of
it), the change will be reflected in the slice. A slice of a list in
Python, OTOH, constructs a completely independent list.
It may be possible that lists in CPython could be made to share their
internal arrays with other lists on a copy-on-write basis, which could
allow slicing to be O(1) as long as neither list is modified while the
array is being shared. I expect this would be a substantial piece of
work, and I don't know if it's something that anybody has looked into.
This is what I was after. Thank you Ian.
So we basically don't have a view of a list. Makes sense now, since
slice does create a new list. I should have seen that. Thank you once
again.
It would a good addition though. Even if only on specialized
implementations like pypy. Inspecting and not changing lists is a good
chunk of our code. But that's besides the point. I was more interested
in understanding the behavior. Thank you once again.
Not directly affecting slices but the idea of views has come into Python
3, please see:-
https://www.python.org/dev/peps/pep-3118/
https://docs.python.org/3/whatsnew/3.3.html#pep-3118-new-memoryview-implementation-and-buffer-protocol-documentation
https://docs.python.org/3/library/stdtypes.html#typememoryview
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Mark Lawrence
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