On Mon, Dec 10, 2018 at 5:23 AM E. Madison Bray <erik.m.b...@gmail.com> wrote:
> Indeed; I believe it is very useful to have a map-like object that is > effectively an augmented list/sequence. but what IS a "map-like object" -- I'm trying to imagine what that actually means. "map" takes a function and maps it onto a interable, returning a new iterable. So a map object is an iterable -- what's under the hood being used to create it is (and should remain) opaque. Back in the day, Python was "all about sequences" -- so map() took a sequence and returned a sequence (an actual list, but that's not the point here). And that's pretty classic "map". With py3, there was a big shift toward iterables, rather than sequences as the core type to work with. There are a few other benefits, but the main one is that often sequences were made, simply so that they could be immediately iterated over, and that was a waste of resources. for i, item in enumerate(a_sequence): ... for x, y in zip(seq1, seq2): ... These two are pretty obvious, but the same approach was taken over much of python: dict.keys(), map(), range(), .... So now in Python, you need to decide, when writing code, what your API is -- does your function take a sequence? or does it take an iterable? Of course, a sequence is an iterable, but a iterable is not (necessarily) a sequence. -- so back in the day, you din't really need to make the decision. So in the case of the Sage example -- I wonder what the real problem is -- if you have an API that requires a sequence, on Py2, folks may have well been passing it the result of a map() call. -- note that they weren't passing a "map object" that is now somehow different than it used to be -- they were passing a list plain and simple. And there are all sorts of places, when converting from py2 to py3, where you will now get an iterable that isn't a proper sequence, and if the API you are using requires a sequence, you need to wrap a list() or tuple() or some such around it to make the sequence. Note that you can write your code to work under either 2 or 3, but it's really hard to write a library so that your users can run it under either 2 or 3 without any change in their code! But note: the fact that it's a map object is just one special case. I suppose one could write an API now that actually expects a map object (rather than a generic sequence or iterable) but it was literally impossible in py2 -- there was no such object. I'm still confused -- what's so wrong with: list(map(func, some_iterable)) if you need a sequence? You can, of course mike lazy-evaluated sequences (like range), and so you could make a map-like function that required a sequence as input, and would lazy evaluate that sequence. This could be useful if you weren't going to work with the entire collection, but really wanted to only index out a few items, but I'm trying to imagine a use case for that, and I haven't. And I don't think that's the use case that started this thread... -CHB > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov
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