On Thu, May 27, 2021 at 11:31 PM Ethan Furman firstname.lastname@example.org 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.
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.