[Python-ideas] __iter__(), keys(), and the mapping protocol

Elias Tarhini eltrhn at gmail.com
Thu Sep 13 01:50:03 EDT 2018

 On Wed, Sep 12, 2018 at 4:42 PM Michael Selik <mike at selik.org> wrote:

> You want to have a Mapping that does not supply a keys method? What use
> case motivated your proposal?

Yes, my proposal was to consider allowing __iter__() to subsume keys()
entirely, for the reasons outlined in my second email -- which I'm just
realizing was an accidental "reply one" to Alex Walters rather than a
"reply all", yikes! Here it is, duplicated:

Ahh, no, I phrased my question a bit badly -- I'm not proposing that the
> keys() method be *removed* from what it exists on currently, but rather
> that this:
> > To be treated like a mapping everywhere, python requires that you
> define* a keys() method, so why not use it?
> be changed, such that only __iter__() and __getitem__() indicate a mapping
> rather than keys() and the same. The latter method would then be
> optional, but not necessarily removed from the front-facing API.
> Granted, my only strong argument is that the ** unpacking operator depends
> on this method to do its job, and it's currently alone amongst Python's
> operators in depending on a non-dunder to do so; that, combined with the
> point heading the below paragraph, is also why I'm only going after keys()
> — not items() or sequences' index() or any of the other
> normally-named-but-important methods.
> And given that __iter__() is already recommended to iterate over a
> mapping's keys, it doesn't seem illogical to me to just let it take that
> job officially (and in turn, I guess, to make keys() on stdlib/built-in
> types just return __iter__()). I don't believe there would be another
> expected use for it in a mapping, anyway, correct? (I haven't done the
> research here, but there could be third-party authors who use it to iterate
> over items(), given that “why does it iterate over just keys” a
> somewhat-recognizable discussion topic on such beginner forums as
> /r/learnpython... if so, this could be a good opportunity to
> force-standardize that behavior as well.)
> Does this clarify my intent? ...I don't doubt that there are roadblocks
> I'm still ignoring, though.
> Eli

However, Serihy's example of {**[0, 2, 1]} is so-plainly irreconcilable --
somewhat embarrassing for me to have missed it -- that I'm now reluctantly
no longer in favor. (Well, really, I'm tempted to say *why not?*, but I do
see that it wouldn't be a good thing overall.)
And I still kind of feel that there should be a dunder involved somewhere
in this, but nowhere near strongly enough to dispute that *"[t]he dunder
methods are dunder methods because they are not generally directly useful.
[There doesn't seem to be] a major problem with having the mapping api call
keys() [...]"*, as it's reasonable and rationalizes the current system well
enough. Thank you for bearing with ;)


On Wed, Sep 12, 2018 at 4:42 PM, Michael Selik <mike at selik.org> wrote:

> Elias,
> I'm a little confused about what you're suggesting. You want to have a
> Mapping that does not supply a keys method? What use case motivated your
> proposal?
> On Mon, Sep 10, 2018, 7:04 PM Elias Tarhini <eltrhn at gmail.com> wrote:
>> This has been bouncing around in my head for a while regarding the
>> requisite keys() method on mappings:
>> How come the ** unpacking operator, a built-in language feature, relies
>> on a non-dunder to operate?
>> To me, I mean to say, requiring that classes implement keys() – a method
>> whose name is totally undistinguished – in order to conform to the mapping
>> protocol feels like a design running counter to Python's norm of using
>> dunders for everything "hidden". I am not sure if it feels dirty to anybody
>> else, however. Interestingly, the docs already say
>> <https://docs.python.org/3/reference/datamodel.html#object.__iter__>
>> that *[f]or mappings, [__iter__()] should iterate over the keys of the
>> container*, but it of course is not enforced in any way at present.
>> So, then — how about enforcing it? Should __iter__(), for the reasons
>> above, replace the current purpose of keys() in mappings?
>> I'm not properly equipped at the moment to mess around with CPython
>> (sorry), but I assume at a minimum this would entail either replacing all
>> instances of PyMapping_Keys() with PyObject_GetIter() or alternatively
>> changing  PyMapping_Keys() to call the latter.
>> Does it sound like a reasonable change overall?
>> Eli
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