itemgetter with default arguments

[Serhiy Storchaka]

04.05.18 20:04, Steven D'Aprano пише:
> On Fri, 04 May 2018 09:17:14 -0600, Ian Kelly wrote:
>> On Fri, May 4, 2018 at 7:01 AM, Steven D'Aprano
>> <steve+comp.lang.python at pearwood.info> wrote:
>>> Here are the specifications:
>>>
>>> * you must use lambda, not def;
>>
>> Why? This seems like an arbitrary constraint.
> 
> You'll have to ask the two core devs. In my post, in the part you
> deleted, I wrote:
> 
>      Two senior developers have rejected this feature, saying
>      that we should just use a lambda instead.
> 
> 
> My guess is that they were thinking that there's no need to complicate
> itemgetter for this use-case when it is just as easy to write up a quick
> lambda to do the job.

I prefer using local functions instead of lambdas, but in many concrete 
cases it is possible to use a lambda.

Consider a concrete example. You need to sort a list of 2- or 3- element 
tuples by the first and third items (third items are strings if 
presented). itemgetter(0, 2) doesn't work because some tuples has only 2 
items. But you can use the following lambdas:

     lambda t: (t[0], t[2] if len(t) > 2 else '')

     lambda t: (t[0], t[2]) if len(t) > 2 else (t[0],)

     lambda t: (t[0], (t + ('',))[2])

     lambda t: t[:1] + t[2:]

The second and the forth options support also the case when there is no 
natural minimal value for third items (e.g. for negative integers) or if 
you want to order 2-tuples before 3-tuples with empty third item and the 
same first item. This isn't possible with itemgetter with default arguments.

If 2-tuples are pretty rare, it may be more efficient to use the 
following function:

     def sortkey(t):
         try:
             return t[0], t[2]
         except IndexError:
             return t[0],