[Python-ideas] keyword-only args in __getitem__

[Nick Coghlan]

On 12 Apr 2014 20:54, "Chris Angelico" <rosuav at gmail.com> wrote:
>
> 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.
>
> 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. 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. 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! :)

A getitem() builtin seems like a more obvious parallel with getattr(), and
pairs up nicely with operator.itemgetter.

Proposals of additional special cases like this is a reasonable outcome of
the rejection of the except expressions PEP.

Cheers,
Nick.

>
> ChrisA
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