random.shuffle slow on deque

[phr]

BPO	4123
Nosy	@loewis, @birkenfeld, @rhettinger, @pitrou, @benjaminp
Files	dequedoc.diff: Doc patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2008-10-16.18:53:40.420>
created_at = <Date 2008-10-14.22:03:00.543>
labels = ['type-feature', 'docs']
title = 'random.shuffle slow on deque'
updated_at = <Date 2008-10-16.18:53:40.362>
user = 'https://bugs.python.org/phr'

bugs.python.org fields:

activity = <Date 2008-10-16.18:53:40.362>
actor = 'benjamin.peterson'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2008-10-16.18:53:40.420>
closer = 'benjamin.peterson'
components = ['Documentation']
creation = <Date 2008-10-14.22:03:00.543>
creator = 'phr'
dependencies = []
files = ['11810']
hgrepos = []
issue_num = 4123
keywords = ['patch']
message_count = 7.0
messages = ['74773', '74776', '74833', '74834', '74839', '74843', '74844']
nosy_count = 6.0
nosy_names = ['loewis', 'georg.brandl', 'rhettinger', 'phr', 'pitrou', 'benjamin.peterson']
pr_nums = []
priority = 'low'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue4123'
versions = ['Python 2.6', 'Python 3.0']

[phr]

This is observed in Python 2.5.1, I haven't tried any later versions.

d = collections.deque(xrange(100000))
random.shuffle(d)

is quite slow. Increasing the size to 200k, 300k, etc. shows that the
runtime increases quadratically or worse. It's much faster to convert
the deque to a list, shuffle the list, and make a new deque from the
shuffled list.

[loewis]

Why do you think this is a bug?

[phr]

If it's not a bug, it is at least a surprising gotcha that should be
documented in the manual. The collections module is described in the
library docs as "high performance container datatypes" but I could not
possibly consider the observed behavior to be high performance.

[pitrou]

Well, perhaps the deque documentation should make it clear that random
access is O(n), rather than O(1) for a list. With this information it is
easy to infer that operations such as shuffle() can be much slower on a
deque.

[rhettinger]

Will add a note to the deque docs that random access is O(n).

[rhettinger]

Patch attached.

[benjaminp]

Done in r66913.