On Mon, Aug 3, 2020 at 2:35 PM Todd <toddr...@gmail.com> wrote: > On Mon, Aug 3, 2020, 17:13 Guido van Rossum <gu...@python.org> wrote: > >> On Mon, Aug 3, 2020 at 1:49 PM Christopher Barker <python...@gmail.com> >> wrote: >> >>> >>> Yes, that would be correct. However, the function could instead be >>>> defined as: >>>> >>>> def __getitem__(self, index, /, **kwargs): >>>> ... >>>> >>>> and then there'd be no conflict (as "self" and "index" must be passed >>>> positionally). In effect, the naive spelling (which permits self and >>>> index to be passed as keywords) would be a subtle bug that could >>>> easily be corrected. >>>> >>> >>> sure, but it would be a bug in a LOT of existing code! >>> >>> I wonder, if this were to be introduced, if the interpreter could have a >>> special case that would call __getitem__ in a special way to avoid this bug >>> in old code. >>> >> >> Good edge case to consider. But would it really be such a problem? If you >> have an existing class like this: >> >> class C: >> def __getitem__(self, index): ... >> >> c = C() >> >> then presumably calling `c[1, index=2]` would just be an error (since it >> would be like attempting to call the method with two values for the `index` >> argument), and ditto for `c[1, 2, 3, index=4]`. The only odd case might be >> `c[index=1]` -- but presumably that would be equivalent to `c[(), index=1]` >> so it would still fail. >> > > The problem is that knowing what labels are not allowed would require > knowing the implementation details of the dunder methods of each individual > class, which I don't think is typically considered public API as far as I > am aware. > >> I don't understand what you're arguing; I think you didn't follow my argument here completely.
My point is that all existing `__getitem__` implementations will raise errors if any keywords are given, even if the keyword happens to correspond to the name of the argument (say, `index`). This is to counter Chris B's concern that if an existing `__getitem__` implementation didn't use the '/' notation to indicate that `self` and `index` are positional, it would have a bug. I claim that the bug will *only* be present when someone adds keyword support to their `__getitem__` method without using the '/'. Since that's not existing code, this "proves" that adding this feature would not introduce a subtle bug in a lot of existing code -- only in carelessly written new (or updated) code. -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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