[Python-ideas] keyword-only args in __getitem__

[Joseph Martinot-Lagarde]

Le 13/04/2014 02:45, Chris Angelico a écrit :
> On Sun, Apr 13, 2014 at 5:28 AM, Joseph Martinot-Lagarde
> <joseph.martinot-lagarde at m4x.org> wrote:
>> The main use case is to be able to set a default value for indexing. I
>> though about it after reading PEP 463 (Exception-catching expressions), this
>> would be an alternative for one of the problem this PEP is trying to solve.
>> Compare these solutions:
>>>>> lst = [1, 2, 3]
>>>>> value = lst[3]
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> IndexError: list index out of range
>>>>> try:  # Python
>> ....     value = lst[3]
>> .... except IndexError:
>> ....     value = 0
>>>>> value = lst[3] except IndexError: 0  # PEP 463
>>>>> value = lst[3, default=0]  # My proposal
>
> An interesting idea, but that looks far too much like a function call.
> Square brackets and then more and more a function call inside... may
> fail the grit test.

I tend to see square brackets as a function call but "indexing 
oriented". Using parenthesis would correspond to calling __call__ while 
using brackets corresponds to calling __getitem__. And sometimes I'd 
like to be able to use I:J:K notation for slices in function calls...

>
> Part of the point of except expressions was that the implementation of
> __getitem__ wouldn't need to have two branches in it. Currently, you
> need to write __getitem__ (which raises an exception on finding a
> problem) plus something else, eg get(), which returns a default
> instead. By your proposal, both branches would go inside __getitem__,
> which means they could share code; but there still need to be two
> branches. That's two branches that need to be written, tested, etc,
> and if the author hasn't thought to handle default=, there's no way to
> get that functionality, same as if there's no get() method.

Yes the code would be included by the author rather than by the user. 
Part of the success of python is that it comes with batteries included !

> And just
> as with the get method, there'll be an ad-hoc and fairly arbitrary
> puddle of names (some will go "default=", others will say that's way
> too long and go "def=", except that that's a keyword so they'll use
> "dflt=" or something...), unless there's a strong force pushing people
> to one consistent name.

Well, this argument would hold against using keywords arguments in any 
function call.

> Terry's suggestion solves that part of it, but
> also restricts the feature to just default values; with a more general
> keyword argument system, it would make sense to also use this with
> __setitem__ and __delitem__, and I'm not sure how "default value"
> should be interpreted there. Which is better is, of course, a matter
> of opinion! :)

setitem and delitem don't look like function calls at all IMHO. Keywords 
arguments for them would be less useful than for getitem.

>
> ChrisA
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