[issue14050] Tutorial, list.sort() and items comparability

[Raymond Hettinger]

Change by Raymond Hettinger :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue14050] Tutorial, list.sort() and items comparability

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset cb8de91dadf15925fb95069cb190398e1d485f56 by Raymond Hettinger 
(Miss Islington (bot)) in branch '3.8':
bpo-14050: Note that not all data can be sorted (GH-15381) (GH-15395)
https://github.com/python/cpython/commit/cb8de91dadf15925fb95069cb190398e1d485f56


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[issue14050] Tutorial, list.sort() and items comparability

[miss-islington]

Change by miss-islington :


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pull_requests: +15103
pull_request: https://github.com/python/cpython/pull/15395

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[issue14050] Tutorial, list.sort() and items comparability

[Raymond Hettinger]

Raymond Hettinger  added the comment:


New changeset 4109263a7edce11194e301138cf66fa2d07f7ce4 by Raymond Hettinger in 
branch 'master':
bpo-14050: Note that not all data can be sorted (GH-15381)
https://github.com/python/cpython/commit/4109263a7edce11194e301138cf66fa2d07f7ce4


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[issue14050] Tutorial, list.sort() and items comparability

[Raymond Hettinger]

Change by Raymond Hettinger :


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pull_requests: +15091
pull_request: https://github.com/python/cpython/pull/15381

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[issue14050] Tutorial, list.sort() and items comparability

[Martin Panter]

Martin Panter added the comment:

The first paragraph in the patch already seems to have been applied, for Issue 
21575.

The Sorting How-to 
https://docs.python.org/dev/howto/sorting.html#odd-and-ends already 
guarantees that defining only __lt__() is sufficient for sorted() and 
list.sort(). And the list.sort() specification 
https://docs.python.org/dev/library/stdtypes.html#list.sort says “only  
comparisons” are used, which implies that only __gt__() may also be sufficient.

It might be good to change “ordering relationship” to “total ordering 
relationship”, but I think further explanation and other details are probably 
not worth adding to the tutorial. They could be clarified in the main 
documentation, but that is probably a separate issue.

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nosy: +vadmium

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[issue14050] Tutorial, list.sort() and items comparability

[R. David Murray]

R. David Murray added the comment:

Unless I'm misremembering, it is exactly __lt__ (or __gt__, if __lt__ returns 
NotImplemented) that sorting depends on.  Since I'm sure there is code out 
there that depends on this fact, I wonder if it should be part of the language 
definition.

Also, the comparison documentation 
(https://docs.python.org/3/reference/expressions.html#comparisons) speaks about 
total ordering as being the requirement, which has a specific mathematical 
meaning (which sets, for example, do not satisfy, even though they have a 
__lt__ method).  Whether or not the distinction is worth explaining in the 
tutorial is a open question.

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[issue14050] Tutorial, list.sort() and items comparability

[Andy Maier]

Andy Maier added the comment:

Comments on v2 of both patches:

1. The paragraph Each item needs to ... describes the requirement in terms of 
ordering relationships between items. It would be both simpler and less 
ambiguous to describe the requirement in terms of type must be orderable. See 
the text added to sorted() as part of the patch for issue10289, for details.

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[issue14050] Tutorial, list.sort() and items comparability

[Andy Maier]

Andy Maier added the comment:

Uploaded patch version py34_v2, which contains the following changes relative 
to 3.4:

1. The changes in the description of list.sort() from default in list.sort(), 
by adding this text:
  (the arguments can be used for sort customization, see :func:`sorted` for 
their explanation)

2. The proposed extension of the description of list.sort() from patch version 
py32.

3. A statement that TypeError is raised if the ordering relationship is not 
established.

4. A reference where to look in order to establish ordering relationship for 
user-defined classes. (referencing the Basic customization section of the 
Language Reference).

5. A reference where the ordering relationships for built-in types are 
described (referencing the Comparison chapter of the Language Reference).

- Please review.

Andy

--
Added file: http://bugs.python.org/file35854/issue14050-list_sort-py34_v2.diff

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[issue14050] Tutorial, list.sort() and items comparability

[Andy Maier]

Andy Maier added the comment:

uploaded patch version py27_v2, which contains the same changes as py34_v2, 
relative to 2.7, except for this differences:

1. The change from default was already in 2.7.

2. The reference to defining ordering methods for user-defined classes includes 
a reference to object.__cmp__(), in addition.

- Please review

Andy

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Added file: http://bugs.python.org/file35855/issue14050-list_sort-py27_v2.diff

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[issue14050] Tutorial, list.sort() and items comparability

[Andy Maier]

Andy Maier added the comment:

Just out of curiosity: Why do the patches attached to this issue not have a 
review link?

Also, both (2.7 and 3.2) patches do not line up with the current 2.7 and 3.x 
tip, both hunks get rejected.

Comments on both patches:

1. It would be helpful if the text Each item needs to define an ordering 
relationship. was followed by a statement about what happens when that is not 
the case (that is what Ezio also suggested), and where to look for details on 
how to define an ordering relationship. The problem with that is that there is 
no good place that is devoted to exactly that. The relatively best place for 
defining comparison I found so far is: 
https://docs.python.org/2.7/tutorial/datastructures.html#comparing-sequences-and-other-types
 and its 3.x equivalent.

2. The example that raises the TypeError is not needed, IMHO. This puts too 
much focus on the case that does not work. If we mention in the description 
that a TypeError is raised, that should be sufficient.

Andy

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[issue14050] Tutorial, list.sort() and items comparability

[Ezio Melotti]

Ezio Melotti added the comment:

 Why do the patches attached to this issue not have a review link?

because

 both (2.7 and 3.2) patches do not line up with the current
 2.7 and 3.x tip, both hunks get rejected.

The review link only appears if the patch applies cleanly on the default 
branch.  Preparing a new patch against default should make the link appear.

--
type:  - enhancement
versions: +Python 3.4, Python 3.5 -Python 3.2, Python 3.3

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[issue14050] Tutorial, list.sort() and items comparability

[Andy Maier]

Andy Maier added the comment:

Ah! I was somehow suspecting that. Thanks for clarifying!

I'll prepare a patch.

To correct my earlier message, the best place to link for comparisons is 
probably the Conparisons subchapter of the Expressions chapter in the 
reference. See also issue12067.

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[issue14050] Tutorial, list.sort() and items comparability

[R. David Murray]

R. David Murray added the comment:

The review link will also appear if the patch is generated via hg diff with 
diff.git turned *off*.  You will note that both existing patches use --git, 
which omits parent changeset information, so our system can't figure out what 
to apply them against in order to generate the review link.

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nosy: +r.david.murray

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[issue14050] Tutorial, list.sort() and items comparability

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


--
assignee: docs@python - rhettinger
nosy: +rhettinger

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[issue14050] Tutorial, list.sort() and items comparability

[Tshepang Lekhonkhobe]

Tshepang Lekhonkhobe tshep...@gmail.com added the comment:

An an aside, shouldn't that be in-place instead of in place 
(http://en.wikipedia.org/wiki/In-place)?

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[issue14050] Tutorial, list.sort() and items comparability

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

Nope, the expression would be hyphenated only when used as an adjective:

- “This sorts the sequence in place”
vs.
‑ “Performs an in-place rearrangement by birthdate”

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[issue14050] Tutorial, list.sort() and items comparability

[Ezio Melotti]

Ezio Melotti ezio.melo...@gmail.com added the comment:

We could just mention that a TypeError is raised if some of the elements can't 
be compared. (Note that sorting a list that contains some types that cannot be 
compared might still succeed in some corner cases, but there's no reason to 
mention this).

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nosy: +ezio.melotti

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[issue14050] Tutorial, list.sort() and items comparability

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

If I had read your patched version when I was a beginner, I don’t think it 
would have helped me much.  (Sorry for not coming up with an alternative right 
now, it’s late/early here and I’m tired :)

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nosy: +eric.araujo, terry.reedy

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[issue14050] Tutorial, list.sort() and items comparability

[Sandro Tosi]

New submission from Sandro Tosi sandro.t...@gmail.com:

I'm providing patches for what reported at 
http://mail.python.org/pipermail/docs/2012-February/007481.html . I'm not sure 
on wording or even if we want them in the tutorial section, but I think it 
would be nice to have it documented nonetheless.

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assignee: docs@python
components: Documentation
files: list_sort-py27.diff
keywords: patch
messages: 153648
nosy: docs@python, sandro.tosi
priority: normal
severity: normal
stage: patch review
status: open
title: Tutorial, list.sort() and items comparability
versions: Python 2.7, Python 3.2, Python 3.3
Added file: http://bugs.python.org/file24557/list_sort-py27.diff

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[issue14050] Tutorial, list.sort() and items comparability

[Sandro Tosi]

Changes by Sandro Tosi sandro.t...@gmail.com:


Added file: http://bugs.python.org/file24558/list_sort-py32.diff

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