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 <rosuav@gmail.com> wrote:
On Sat, Dec 30, 2017 at 2:38 AM, Steven D'Aprano <steve@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
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