[Python-ideas] Accepting keyword arguments for __getitem__

[Terry Reedy]

On 6/23/2014 8:06 AM, Stefano Borini wrote:
> Dear all,
>
> At work we use a notation like LDA[Z=5] to define a specific level
 > of accuracy for our evaluation. This notation is used
> just for textual labels, but it would be nice if it actually worked
 > at the scripting level, which led me to think to the following:
> at the moment, we have the following
>
>>>> class A:
> ...     def __getitem__(self, y):

This actually says that y can be passed by position or name ;-)

> ...         print(y)
> ...
>>>> a=A()
>>>> a[2]
> 2

 >>> a.__getitem__(y=2)
2

>>>> a[2,3]
> (2, 3)
>>>> a[1:3]
> slice(1, 3, None)
>>>> a[1:3, 4]
> (slice(1, 3, None), 4)
>>>>
>
> I would propose to add the possibility for a[Z=3], where y would then be a
> dictionary {"Z": 3}.  In the case of a[1:3, 4, Z=3, R=5], the value of y would
> be a tuple containing (slice(1,3,None), 4, {"Z": 3}, {"R": 5}). This allows to
> preserve the ordering as specified (e.g. a[Z=3, R=4] vs a[R=4, Z=3]).

As others have pointed out, you are not actually asking that __getitem__ 
'accept keyword arguments'.  Rather you are asking that "x=y" be seen as 
an abbreviation for "{'x':y}" in a very rare usage in a particular 
context to save 4 (admittedly awkward) keystrokes. The resulting 
confusion is not worth it. Saving 4 of 7 might seem worth it, but it 
real cases, like "precision=4" versus "{'precision':4}" the ratio is 
lower. I also wonder whether you might sometimes us the same spec in 
multiple subscriptings, so that you might define "p = {'precision': 4}" 
once and use it multiple times.

In your introductory paragraph, you only specify one optional parameter 
-- accuracy. So it is not clear why you do not just write a .get(self, 
ob, accuaracy=default) method. If their are multiple options, make them 
keyword only.

-- 
Terry Jan Reedy