[issue11107] Cache constant "slice" instances

[Stefan Behnel]

New submission from Stefan Behnel <scoder at users.sourceforge.net>:

Follow-up to ticket 10227. The following facts seem to indicate that it would be worth caching constant instances of the slice type, such as in [:] or [:-1].

with cached slice instance:

$ ./python -m timeit -s 'l = list(range(100)); s=slice(None)' 'l[s]'
1000000 loops, best of 3: 0.464 usec per loop
$ ./python -m timeit -s 'l = list(range(10)); s=slice(None)' 'l[s]'
10000000 loops, best of 3: 0.149 usec per loop
$ ./python -m timeit -s 'l = list(range(10)); s=slice(None,1)' 'l[s]'
10000000 loops, best of 3: 0.135 usec per loop

uncached normal usage:

$ ./python -m timeit -s 'l = list(range(100))' 'l[:]'
1000000 loops, best of 3: 0.499 usec per loop
$ ./python -m timeit -s 'l = list(range(100))' 'l[:1]'
10000000 loops, best of 3: 0.171 usec per loop

Timings based on Python 3.2 rc2.

A quick grep against the py3k stdlib finds 2096 lines in 393 files that use constant slices.

----------
components: Interpreter Core
messages: 127804
nosy: scoder
priority: normal
severity: normal
status: open
title: Cache constant "slice" instances
type: performance
versions: Python 3.3

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11107>
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