Add list.sorted()

[rhettinger]

BPO	825814
Nosy	@aleaxit, @rhettinger
Files	cs.diff: Patch 1 with docs, unittests, and annoucement cs2.diff: Patch 2 avoids unnecessary copy cs3.diff: Patch 3 with better argument handling sorted.diff: Patch 4 implements class method list.sorted()

^{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 = None
closed_at = <Date 2003-10-29.09:46:06.000>
created_at = <Date 2003-10-18.00:34:53.000>
labels = ['interpreter-core']
title = 'Add list.sorted()'
updated_at = <Date 2003-10-29.09:46:06.000>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2003-10-29.09:46:06.000>
actor = 'aleax'
assignee = 'none'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2003-10-18.00:34:53.000>
creator = 'rhettinger'
dependencies = []
files = ['5645', '5646', '5647', '5648']
hgrepos = []
issue_num = 825814
keywords = ['patch']
message_count = 6.0
messages = ['44800', '44801', '44802', '44803', '44804', '44805']
nosy_count = 2.0
nosy_names = ['aleax', 'rhettinger']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue825814'
versions = ['Python 2.4']

[rhettinger]

As discussed on py-dev, this returns a sorted copy of a
list while leaving the original intact.

[rhettinger]

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Revised the patch to avoid an unnecessary copy whenever the
underlying object is not referenced anywhere else -- this
occurs in the common use case: for elem in
d.keys().copysort(): . . .

[rhettinger]

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Rationale for the change
---------------------
An inline sort is usable anywhere an expression is allowed.
This includes important places like function call arguments
and list comprehensions:

  todo = [t for t in tasks.copysort() if due_today(t)]

  genhistory(date, events.copysort(key=incidenttime))

Spreading these out over multiple lines is an unnecessary
distractor from the problem domain, making the code harder
to understand and maintain.

Also, using copysort() eliminates a unnecessary variable
that changes state from unsorted to sorted and has a
lifetime longer than the data is actually needed. In longer
code fragments, this decreases code complexity, code length,
the number of variables, and decreases the risk of using a
variable in the wrong state which is a common source of
programming errors.

[rhettinger]

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Alex, can you give this patch a second review.

I did not put in additional classmethod warnings because all
plausible misunderstandings raise an error anyway.

[rhettinger]

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Applied.

[aleaxit]

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Looks good to me! I understand not wanting -- given the clean and
spare code for list.sorted -- to go to the considerable trouble needed
to give more specific error messages; however, if we release like this
we WILL get questions about it on c.l.py and help@python.org -- "why doesn't ''for x in mydict.keys().sorted():'' work?!" etc. Do you mind if
eventually I _do_ try to ensure the error messages specifically mention list.sorted being a classmethod and requiring one iterable argument?