On Mon, Nov 08, 2021 at 01:43:03PM -0800, Ethan Furman wrote:
> When is an empty container contained by a non-empty container?
[...]
> SomeFlag.nothing in SomeFlag.something <-- ???
I don't think that consistency with other containers is particularly
relevant here. More useful is consistency with other flag objects.
What's SomeFlag? I presume it is some sort of Enum, or bitset.
Presumably SomeFlag.nothing is equivalent to a bitset of all zeroes.
(If it means something else, then I have no clue what to suggest.) You
might have:
something = 0b11010
0 in something # ???
1 in something # False
2 in something # True
4 in something # False
8 in something # True
16 in something # True
32 in something # False
So much is obvious. But what about `3 in something`?
If that is interpreted as an *all* operation, you get:
3 in something --> all(i in something for i in (1, 2)) # False
but if it is an *any* operation:
3 in something --> any(i in something for i in (1, 2)) # True
I don't mean to imply that you must actually use all/any in your
implementation. I mean only that it is conceptually equivalent to ANDing
each of the flags (all) versus ORing the flags (any).
*If* that is how you interpret your containment tests, that implies a
natural interpretation for `nothing in something`. The vacuous truth of
all is True, and of any is False:
0 in something --> all(i in something for i in ()) # True
0 in something --> any(i in something for i in ()) # False
Vacuous truth is not the only possible interpretation. If you have some
other obvious and natural interpretation of `0 in something` then you
probably wouldn't be asking here, but if you did, you could follow that
interpretation. With a good reason to violate the vacuous truth rules,
it would only be a *little* surprising.
--
Steve
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