[Python-Dev] Function to handle None in sort operation, was Re: python 3 niggle: None < 1 raises TypeError

[Nick Coghlan]

On 14 February 2014 22:01, Peter Otten <__peter__ at web.de> wrote:
> M.-A. Lemburg wrote:
>
>> IMO, it was a mistake to have None return a TypeError in
>> comparisons, since it makes many typical data operations
>> fail, e.g.
>>
>> Python2:
>>>>> l = [1,2,None,4,5,None,6]
>>>>> l.sort()
>>>>> l
>> [None, None, 1, 2, 4, 5, 6]
>>
>> Python3:
>>>>> l = [1,2,None,4,5,None,6]
>>>>> l.sort()
>> Traceback (most recent call last):
>>   File "<stdin>", line 1, in <module>
>> TypeError: unorderable types: NoneType() < int()
>
> While it is trivial to fix
>
>>>> sorted([1,2,None,4,5,None,6],
> ... key=lambda x: (x is None, x))
> [1, 2, 4, 5, 6, None, None]
>
> maybe the key should be given a name like functools.none_first/none_last in
> order to offer a standard approach?

Yeah, I was thinking something similar. I posted an RFE if anyone
wants to run with the idea for Python 3.5:
http://bugs.python.org/issue20630 (it's also the kind of thing that is
quite amenable to inclusion in libraries like six and future for
Python 2/3 compatible code bases - in Python 2, it can just be a pass
through, while in Python 3 it can actually alter the sort operation to
allow None values).

At a broaded scale, this thread made me realise it may be worth
defining a __key__ protocol so custom comparisons are easier to
implement correctly: http://bugs.python.org/issue20632

That would need a PEP, but currently, there are lots of things you
need to deal with around hashability, comparison with other types, etc
when attempting to define a custom ordering. A new __key__ protocol
could provide sensible default behaviour for all of those, such that
you only needed to define the other methods if you wanted to override
the defaults for some reason. (As noted in the issue, this is also
amenable to being implemented as a third party module prior to
proposing it as a language feature)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia