We already have a built-in immutable set for Python. It's called frozenset. On Fri, Dec 29, 2017 at 10:56 AM Chris Angelico <ros...@gmail.com> wrote:
> On Sat, Dec 30, 2017 at 2:38 AM, Steven D'Aprano <st...@pearwood.info> > wrote: > > The lack of support for the `in` operator is a major difference, but > > there's also `len` (equivalent to "count the one bits"), superset > > and subset testing, various in-place mutator methods, etc. Java has a > > BitSet class, and you can see the typical sorts of operations > > commonly required: > > > > https://docs.oracle.com/javase/8/docs/api/java/util/BitSet.html > > Okay. A subclass of int could easily add a few more. Counting the 1 > bits isn't difficult; superset and subset testing are actually the > same as 'contains' but with more than one bit at a time. (In fact, > checking if a set contains a subset is *easier* with ints than with > actual sets!) Are in-place mutators that big a deal? I'm sure there > are sets in languages with no mutables. > > > Of course we can emulate set-like operations using ints, but the > > interfaces are different, which is my point. Here's how to clear all the > > flags of a set or int: > > > > the_flags.clear() > > > > the_flags = 0 # clear all the bits in an int > > That's a consequence of Python's mutability distinction. I don't think > it's a fundamental difference. You could just as easily use "the_flags > = set()" if it weren't for aliasing. > > > Setting a flag is *almost* the same between the two: > > > > the_flags |= {flag} # set > > > > the_flags |= flag # int > > That's because you can implicitly upcast a bitflag to a bitset. > Effectively, ints give you a short-hand that sets can't. But if you > explicitly call BitSet(flag) to create a set containing one flag, it > would have the same effect. > > > although for sets, there are two other ways to set a flag which aren't > > supported by ints: > > > > the_flags.add(flag) > > the_flags.update({flag}) > > > > Similarly for clearing flags: > > > > the_flags.discard(flag) > > > > the_flags & ~flag > > Mutability again. If you were to create an ImmutableSet type in > Python, what would its API look like? My suspicion is that it'd > largely use operators, and that it'd end up looking a lot like the > integer API. > > An integer, at its lowest level, is represented as a set of bits. It's > no more crazy to use an int as a set of bits than to use a string as a > set of characters: > > https://docs.python.org/3/library/stdtypes.html#str.strip > > ChrisA > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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