[Python-Dev] The list.sort(reverse=True) method not consistent with description

[Raymond Bisdorff]

Dear Python developers,

The help(list) shows in a python console the following documentation 
string for the list.sort() method.


sort(self, /, *, key=None, reverse=False)
 |  Sort the list in ascending order and return None.
 |
 |  The sort is in-place (i.e. the list itself is modified) and 
stable (i.e. the

 |  order of two equal elements is **maintained**).

Please notice the following inconsistency in Python3.10.0 and before of 
a sort(reverse=True) result:


>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(reverse=True)
>>> L
>>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

it should be:

>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> reverseTuplesSort(L)
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

Same inconsistency appears when using a sorting key.

Passing easily unnoticed may produce unexpected and potentially wrong 
ranking results.


Best Regards,
RB

--

50, Bvd G.-D. Charlotte
L-1330 Luxembourg
GD Luxembourg (Europe)
(+352) 621 175 242

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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Tim Peters]

[Raymond Bisdorff ]
> ...
> Please notice the following inconsistency in Python3.10.0 and before of
> a sort(reverse=True) result:
>
>  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>  >>> L.sort(reverse=True)
>  >>> L
>  >>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

Looks good to me.

> it should be:
>
>  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>  >>> reverseTuplesSort(L)
> [(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

Stability is irrelevant in the example, because no two list elements
are equal. You appear to be thinking, perhaps, that s[0] == t[0] when
s == (1, 'a') and t == (1, 'c') means s and t "are equal", but that's
not so at all.

>>> s = (1, 'a')
>>> t = (1, 'c')
>>> s == t
False
>>> s < t
True
>>> t > s
True

So s MUST come before t in a forward sort, and t MUST come before s in
a reverse sort.
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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Richard Damon]

On 10/30/21 11:46 AM, Raymond Bisdorff wrote:

Dear Python developers,

The help(list) shows in a python console the following documentation 
string for the list.sort() method.


sort(self, /, *, key=None, reverse=False)
 |  Sort the list in ascending order and return None.
 |
 |  The sort is in-place (i.e. the list itself is modified) and 
stable (i.e. the

 |  order of two equal elements is **maintained**).

Please notice the following inconsistency in Python3.10.0 and before 
of a sort(reverse=True) result:


>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(reverse=True)
>>> L
>>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

it should be:

>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> reverseTuplesSort(L)
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

Same inconsistency appears when using a sorting key.

Passing easily unnoticed may produce unexpected and potentially wrong 
ranking results.


Best Regards,
RB

Why do you thing (1,'a') should be 'bigger' than (1, 'c'), or (2,'b') 
bigger than (2,'d')?


Tuples don't sort on just the first element, but on all their elements 
in order if the previous ones are the same.


--
Richard Damon

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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Evpok Padding]

Hi,

On Sat, 30 Oct 2021 at 16:23, Raymond Bisdorff 
wrote:

>   >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>  >>> L.sort(reverse=True)
>  >>> L
>  >>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]
>
> it should be:
>
>  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>  >>> reverseTuplesSort(L)
> [(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]
>
> No, it should not, because 'd'>'b' and 'c'>'a' ! tuple comparison compares
both elements in lexicographic order.

Cheers,

E
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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Raymond Bisdorff]

Dear All,

I fully agree with your point. By default, all the components of the 
tuple should be used in the comparison.


Yet, I was confused by the following result.
>>> from operator import itemgetter
>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(key=itemgetter(0), reverse=True)
>>> L = [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

Should the tuples comparison is in this case, I thought, not be solely 
based on the first tuple component?

Best Regards

On 10/30/21 18:26, Tim Peters wrote:

[Raymond Bisdorff ]

...
Please notice the following inconsistency in Python3.10.0 and before of
a sort(reverse=True) result:

  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
  >>> L.sort(reverse=True)
  >>> L
  >>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

Looks good to me.


it should be:

  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
  >>> reverseTuplesSort(L)
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

Stability is irrelevant in the example, because no two list elements
are equal. You appear to be thinking, perhaps, that s[0] == t[0] when
s == (1, 'a') and t == (1, 'c') means s and t "are equal", but that's
not so at all.


s = (1, 'a')
t = (1, 'c')
s == t

False

s < t

True

t > s

True

So s MUST come before t in a forward sort, and t MUST come before s in
a reverse sort.



--
50, Bvd G.-D. Charlotte
L-1330 Luxembourg
GD Luxembourg (Europe)
(+352) 621 175 242

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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Tim Peters]

[Raymond Bisdorff ]
> I fully agree with your point. By default, all the components of the
> tuple should be used in the comparison.
>
> Yet, I was confused by the following result.
>  >>> from operator import itemgetter
>  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>  >>> L.sort(key=itemgetter(0), reverse=True)
>  >>> L = [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]
>
> Should the tuples comparison is in this case, I thought, not be solely
> based on the first tuple component?

I'm not sure what you're doing there. The last line in your example
isn't showing the result of sorting, it's assigning a brand new list
to `L`' Here the same thing, but changing the last line to show the
result of sorting:

>>> from operator import itemgetter
>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(key=itemgetter(0), reverse=True)
>>> L
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

So stability does matter when comparing only the tuples' first
components, and tuples with the same first component did retain their
original order.
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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Paul Ganssle]

You are re-assigning the list on line 4 here, not displaying it. I get 
the answer you expect when using the `itemgetter(0)` key:


IPython 7.28.0, on CPython 3.9.7 (default, Aug 31 2021 13:28:12)
>>> import operator
>>> from operator import itemgetter
>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(key=operator.itemgetter(0), reverse=True)
>>> L
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

On 10/30/21 12:47, Raymond Bisdorff wrote:

Dear All,

I fully agree with your point. By default, all the components of the 
tuple should be used in the comparison.


Yet, I was confused by the following result.
>>> from operator import itemgetter
>>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
>>> L.sort(key=itemgetter(0), reverse=True)
>>> L = [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

Should the tuples comparison is in this case, I thought, not be solely 
based on the first tuple component?

Best Regards

On 10/30/21 18:26, Tim Peters wrote:

[Raymond Bisdorff ]

...
Please notice the following inconsistency in Python3.10.0 and before of
a sort(reverse=True) result:

  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
  >>> L.sort(reverse=True)
  >>> L
  >>> [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]

Looks good to me.


it should be:

  >>> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
  >>> reverseTuplesSort(L)
[(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]

Stability is irrelevant in the example, because no two list elements
are equal. You appear to be thinking, perhaps, that s[0] == t[0] when
s == (1, 'a') and t == (1, 'c') means s and t "are equal", but that's
not so at all.


s = (1, 'a')
t = (1, 'c')
s == t

False

s < t

True

t > s

True

So s MUST come before t in a forward sort, and t MUST come before s in
a reverse sort.




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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Raymond Bisdorff]

Dear All, 
You are all completely right. 
Sorry for the confusion. 
Thank you all very much for putting my mind right about this issue. 
Best Regards
Raymond Bisdorff 

> On 30 Oct 2021, at 19:00, Tim Peters  wrote:
> 
> [Raymond Bisdorff ]
>> I fully agree with your point. By default, all the components of the
>> tuple should be used in the comparison.
>> 
>> Yet, I was confused by the following result.
> from operator import itemgetter
> L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
> L.sort(key=itemgetter(0), reverse=True)
> L = [(3, 'e'), (2, 'd'), (2, 'b'), (1, 'c'), (1, 'a')]
>> 
>> Should the tuples comparison is in this case, I thought, not be solely
>> based on the first tuple component?
> 
> I'm not sure what you're doing there. The last line in your example
> isn't showing the result of sorting, it's assigning a brand new list
> to `L`' Here the same thing, but changing the last line to show the
> result of sorting:
> 
 from operator import itemgetter
 L = [(1, 'a'), (2, 'b'), (1, 'c'), (2, 'd'), (3, 'e')]
 L.sort(key=itemgetter(0), reverse=True)
 L
> [(3, 'e'), (2, 'b'), (2, 'd'), (1, 'a'), (1, 'c')]
> 
> So stability does matter when comparing only the tuples' first
> components, and tuples with the same first component did retain their
> original order.

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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Greg Ewing]

On 31/10/21 5:47 am, Raymond Bisdorff wrote:
Should the tuples comparison is in this case, I thought, not be solely 
based on the first tuple component?


Whether you think it should or shouldn't, the fact is that it's not.
This is documented in Section 5.6 of the Library Reference:

"tuples and lists are compared lexicographically by comparing 
corresponding elements. This means that to compare equal, every element 
must compare equal and the two sequences must be of the same type and 
have the same length."


--
Greg
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[Python-Dev] Re: The list.sort(reverse=True) method not consistent with description

[Steven D'Aprano]

On Sun, Oct 31, 2021 at 01:32:29PM +1300, Greg Ewing wrote:
> On 31/10/21 5:47 am, Raymond Bisdorff wrote:
> >Should the tuples comparison is in this case, I thought, not be solely 
> >based on the first tuple component?
> 
> Whether you think it should or shouldn't, the fact is that it's not.
> This is documented in Section 5.6 of the Library Reference:
> 
> "tuples and lists are compared lexicographically by comparing 
> corresponding elements. This means that to compare equal, every element 
> must compare equal and the two sequences must be of the same type and 
> have the same length."

I don't think that applies if you provide a key function. In context, 
Raymond is referring to using itemgetter(0) as the key function, so only 
the first item in the tuple will be considered.

-- 
Steve
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[Python-Dev] Tips on C and the CPython Codebase

[Abdur-Rahmaan Janhangeer]

Greetings list,

I am going to start tinkering with the Python source again (on Linux)
I previously built the source etc using Visual Studio on Windows
Now the EFL ui libs re-ignited my passion for C while playing with
python-efl
And Chris last proposal made me want to re-play with the CPython codebase

I dedicate time for Open Source as a maintainer but also as a learner.
So i'd like to ask so as to maximise my chances playing around with CPython

- Any well grounded C tutorial to recommend? I know CPP but need a
recommendation for C which is easy to follow and goes deep the C way. Or
really any tutorial/s which are helpful like code styles or any series or
...

- Any coding environment which you recommend?

- For devs guide i know there is the official docs, and VStinner's
unofficial
but very helpful guide and a core dev's book published. Is there anything
to know?

- Any more tips around?

I guess sooner or later people figure out things but i'd be very happy to
have some C tips which will certainly provide a speed boost.

Thanks in advance!

Kind Regards,

Abdur-Rahmaan Janhangeer
about  | blog

github 
Mauritius
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