On Thu, May 27, 2021 at 11:31 PM Ethan Furman <ethan@stoneleaf.us> wrote:

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?


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.