itemgetter with default arguments

[Thomas Jollans]

On 2018-05-04 15:01, Steven D'Aprano wrote:
> A re-occurring feature request is to add a default to itemgetter and 
> attrgetter. For example, we might say:
> 
> from operator import itemgetter
> f = itemgetter(1, 6, default="spam")  # proposed feature
> f("Hello World!")  # returns ('e', 'W')
> f("Hello")         # returns ('e', 'spam')
> 
> 
> Two senior developers have rejected this feature, saying that we should 
> just use a lambda instead.
> 
> I might be slow today, but I cannot see how to write a clear, obvious, 
> efficient lambda that provides functionality equivalent to itemgetter 
> with a default value.
> 
> Putting aside the case where itemgetter takes multiple indexes, how about 
> the single index case? How could we get that functionality using a lambda 
> which is simple and obvious enough to use on the fly as needed, and 
> reasonably efficient?
> 
> Here are the specifications:
> 
> * you must use lambda, not def;
> 
> * the lambda must take a single function, the sequence you want to
>   extract an item from;
> 
> * you can hard-code the index in the body of the lambda;
> 
> * you can hard-code the default value in the body of the lambda;
> 
> * if sequence[index] exists, return that value;
> 
> * otherwise return the default value;
> 
> * it should support both positive and negative indices.
> 
> Example: given an index of 2 and a default of "spam":
> 
>     (lambda seq: ... )("abcd") returns "c"
> 
>     (lambda seq: ... )("") returns "spam"

Like this?

>>> spamgetter = (lambda seq, i=2, fallback="spam":
...                  seq[i] if abs(i) < len(seq) or i == -len(seq)
...                  else fallback)
>>> spamgetter("abcd", i=-4)
'a'
>>> spamgetter("abcd")
'c'
>>> spamgetter("")
'spam'
>>>


Making this work for sequences AND dict-like objects is more difficult...


> 
> 
> I might be missing something horribly obvious, but I can't see how to do 
> this using a lambda. I tried using slicing:
> 
>     seq[index:index+1]
> 
> which will return either an empty slice or a one-item slice, but that 
> didn't help me. I feel I'm missing something either obvious, or something 
> impossible, and I don't know which.
> 
> (This isn't a code-golf problem. I care more about being able to do it at 
> all, than about doing it in the minimum number of characters.)
> 
> 
>