On Thu, May 27, 2021 at 11:31 PM Ethan Furman <et...@stoneleaf.us> wrote:
> Greetings! > > The Flag type in the enum module has had some improvements, but I find it > necessary to move one of those improvements > into a decorator instead, and I'm having a hard time thinking up a name. > > What is the behavior? Well, a name in a flag type can be either canonical > (it represents one thing), or aliased (it > represents two or more things). To use Color as an example: > > class Color(Flag): > RED = 1 # 0001 > GREEN = 2 # 0010 > BLUE = 4 # 0100 > PURPLE = RED | BLUE # 0101 > WHITE = RED | GREEN | BLUE # 0111 > > The flags RED, GREEN, and BLUE are all canonical, while PURPLE and WHITE > are aliases for certain flag combinations. But > what if we have something like: > > class Color(Flag): > RED = 1 # 0001 > BLUE = 4 # 0100 > WHITE = 7 # 0111 > > As you see, WHITE is an "alias" for a value that does not exist in the > Flag (0010, or 2). That seems like it's probably > an error. But what about this? > > class FlagWithMasks(IntFlag): > DEFAULT = 0x0 > > FIRST_MASK = 0xF > FIRST_ROUND = 0x0 > FIRST_CEIL = 0x1 > FIRST_TRUNC = 0x2 > > SECOND_MASK = 0xF0 > SECOND_RECALC = 0x00 > SECOND_NO_RECALC = 0x10 > > THIRD_MASK = 0xF00 > THIRD_DISCARD = 0x000 > THIRD_KEEP = 0x100 > > Here we have three flags (FIRST_MASK, SECOND_MASK, THIRD_MASK) that are > aliasing values that don't exist, but it seems > intentional and not an error. > > So, like the enum.unique decorator that can be used when duplicate names > should be an error, I'm adding a new decorator > to verify that a Flag has no missing aliased values that can be used when > the programmer thinks it's appropriate... but > I have no idea what to call it. > > Any nominations? > > -- > ~Ethan~ > > In Math / CompSci there is a definition that almost exactly matches this: Exact Cover - https://en.wikipedia.org/wiki/Exact_cover The difference is that, IIRC, solving the problem is finding and removing all subsets that are unneeded to create an exact cover, so it's kind of arriving at it from a different direction, but 'exact cover' definition itself is a good match. -andrei
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