On Tue, Dec 29, 2020 at 4:24 PM Greg Ewing <greg.ew...@canterbury.ac.nz> wrote:
> The library reference doesn't seem to use the terms "sequence protocol" > and "mapping protocol". It talks about "sequence types" and "mapping > types", but doesn't use the word "protocol" in relation to them. > The only one I've really seen used fairly universally is the "iterator protocol" -- though in the docs, it's under "Iterator Types": https://docs.python.org/3/library/stdtypes.html?highlight=iterable#iterator-types But then, under that: "The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:" So that's (I think) the only place it documented and called a protocol. > Nor is there any clear list of methods that a type needs to provide > in order to be considered a sequence or mapping. Isn't that what the ABCs are? The trick here, in this context, is that something doesn't need to be a fully functioning Mapping to be unpacked. But there are a handful of places where a subset of the Mapping API is needed (apparently .keys() and __getitem__, with a particular relationship). So It is good to (a) be consistent about that -- which I think is the case today, and (b) document it -- which is not the case. Given that where the iteration protocol is described is under built in types, it makes sense to me to t put it there -- maybe call it the "mapping unpacking protocol" (better ideas welcome!). > There are lists of > operations supported by the built-in container types, but those are > fairly extensive, and it's not obvious which ones are vital. > there's more than that -- there's the ABCs. But anyway, the problem with "vital" is the core of duck typing -- what's vital depends on the context -- We can only (and should only) define that for a few places where they are used in core Python -- is there anything else other than Mapping unpacking that we should identify and document? > So are the "sequence protocol" and "mapping protocol" really mythical > beasts that don't really exist? Are they more akin to the infamous > "file-like object" which is whatever it needs to be for the purpose > at hand? > See above -- I don't think there's a Sequence protocol at all. And the "mapping protocol" that at least I"ve been talking about is in a particular context -- unpacking, or somewhat more generally, when you want to be able to access all the items in a Mapping, but not do anything else with it. Guido has since said that the ABCs are intended to be definitive, > but the docs don't really make that clear either. (And the ABC doc page > talks about "APIs", not "protocols"!) > I think the ABCs Are definitive -- but what are they definitive of? They are certainly not minimal protocols. -CHB -- Christopher Barker, PhD Python Language Consulting - Teaching - Scientific Software Development - Desktop GUI and Web Development - wxPython, numpy, scipy, Cython
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