On 01.09.20 17:44, Steven D'Aprano wrote:
(9) Keyword-only subscripts are permitted:obj[spam=1, eggs=2] # calls type(obj).__getitem__(spam=1, eggs=2) del obj[spam=1, eggs=2] # calls type(obj).__delitem__(spam=1, eggs=2) but note that the setter is awkward since the signature requires the first parameter: obj[spam=1, eggs=2] = value # wants to call type(obj).__setitem__(???, value, spam=1, eggs=2) Proposed solution: this is a runtime error unless the setitem method gives the first parameter a default, e.g.: def __setitem__(self, index=None, value=None, **kwargs) Note that the second parameter will always be present, nevertheless, to satisfy the interpreter, it too will require a default value. (Editorial comment: this is undoubtably an awkward and ugly corner case, but I am reluctant to prohibit keyword-only assignment.)
Why does the signature require the first `index` parameter? When I see `obj[spam=1, eggs=2] = value` there's no positional index and so I wouldn't expect one to be passed. Similar to how the following works: >>> def foo(): ... pass ... >>> foo(*()) If there's nothing to unpack nothing will be assigned to any of the parameters. So the following signature would work with keyword-only subscripts: def __setitem__(self, value, **kwargs): I don't how the `[] =` operator is translated to `__setitem__` at the implementation level, so perhaps the no-positional-index case would require yet another opcode, but thinking of it in the following way it would definitely be possible: obj.__setitem__(*(pos_index + (value,)), **kwargs) where `pos_index` is the positional index collected from the `[]` operator as a tuple (and if no such index is given it defaults to the empty tuple). This matches the above no-index signature of `__setitem__`. This is also the signature of the corresponding `__getitem__` and `__delitem__` methods. _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/DDNIG6AWODEMEU22IG4BKLXLXX26YZL4/ Code of Conduct: http://python.org/psf/codeofconduct/
