On 12/07/2013 02:07 AM, Alan Gauld wrote:
On 06/12/13 15:39, spir wrote:
How does slicing in Python really work? Apparently, there are slice
objects (start, past-end, step), generated using either the 'slice'
builtin func or the extended slicing syntax [i:k:s]. Is this correct? [1]
I believe the slice notation simply calls the underlying method as is the case
for most operations in Python. But I'm no expert in the internals, others are
far better qualified.
Does (only) the extended syntax (always) trigger slicing instead of
contructing a new subsequence? (new substring or sublist or whatever,
actually holding a section of the original sequence)
I'm not sure what you think slicing does? But in general a slice produces a new
sequence. Thus
L = [1,2,3,4]
L2 = L[1:2]
L2 is a new list object.
Indeed taking a full slice is one of the commonest ways of making a copy of a
list:
L3 = L[:] # a new copy of L
Are slices and subsequences transparently usable one for the other?
subsequences don't exist as objects in Python so you can't use them in the same
way as a slice (which does explicitly exist). So I don't understand what you
have in mind here.
rationale, design, etc... Maybe this means slices have always existed
and I just missed them totally?
Slices have been in python since I started using it in V1.3. And I think from
before then too. Certainly a very long time.
[2] The fact that slices exist at all shows how worth it is to avoid
needlessly creating subsequences,
No it doesn't. Slices create new sequences.
Creating a "sub sequence" is often the fastest most efficient way to do
something. They are not something to get hung up about unless you have
absolutely proved they are a source of a problem. Especially given the
difficulty of writing reliable in-place code for many sequence operations.
Hum, we are not talking of the same topic, apparently. I mean this, from the
library ref, builtin funcs:
http://docs.python.org/3.3/library/functions.html#slice:
slice(start, stop[, step])
Return a slice object representing the set of indices specified by
range(start, stop, step).
The start and step arguments default to None. Slice objects have read-only
data attributes
start, stop and step which merely return the argument values (or their
default).
They have no other explicit functionality; however they are used by
Numerical
Python and other third party extensions. Slice objects are also generated
when
extended indexing syntax is used. For example: a[start:stop:step] or
a[start:stop, i].
See itertools.islice() for an alternate version that returns an iterator.
Note the third and forelast sentences (extedned syntax apparently means
indicating a step).
spir@ospir:~$ python3
Python 3.3.1 (default, Sep 25 2013, 19:29:01)
[GCC 4.7.3] on linux
Type "help", "copyright", "credits" or "license" for more information.
s = slice(1,-1,3)
s.start, s.stop, s.step
(1, -1, 3)
s
slice(1, -1, 3)
But i don't see what changes below, when using extended indexing syntax:
str = "abcdefghi"
sl = str[1:-1:2]
sl
'bdfh'
type(sl)
<class 'str'>
How to get a actual slice (a view, something like (str, 1, -1, 2) or better
(start-pointer, -1, 2))? Why does slice() exist at all? How to use it? Is it
used internally? Or does python cheat, pretending on the interface, at the
language-side, that this is a str, just to maintain the language's semantics,
while in some cases at least it is not?
Denis
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