On 11/8/21 3:09 PM, Steven D'Aprano wrote:
> On Mon, Nov 08, 2021 at 01:43:03PM -0800, Ethan Furman wrote:
>> 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.
You mean from other programming languages?
> What's SomeFlag? I presume it is some sort of Enum, or bitset.
Yes.
> Presumably SomeFlag.nothing is equivalent to a bitset of all zeroes.
Yes.
> 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
It's an `all()` operation.
> *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.
To rephrase Wikipedia [1]: a vacuously true statement doesn't really say
anything.
I prefer my truths to actually say something. ;-)
I don't know if there's a formal name, but in my mind, if you have something you don't have nothing. If you have a
barrel with nothing in it (not even air, just a hard vacuum) then saying you have nothing in that barrel is a true
statement; as soon as something is in that barrel, then saying you have nothing in the barrel is a false statement.
All of that is academic without a solid (and possibly irritating ;) use-case, so let's look at a different portion of
the standard library -- opening files with `os.open()`:
Help on built-in function open in module posix:
open(path, flags, mode=511, *, dir_fd=None)
Open a file for low level IO. Returns a file descriptor (integer).
A possible `flags` value (and presumably the default value) is `O_RDONLY` which is `0`. I can imagine an implementation
similar to (using integers):
if flags == 0:
stuff
if flags & O_TRUNC == O_TRUNC:
stuff
if flags & O_RDRW == O_RDRW:
stuff
# preliminary stuff done
return do_open_file(...)
If that were converted to use Flag, then the above would still work, but if
written anew could become:
flags = OpenFlag(flags) # in case older code passes in an integer
# ignoring O_RDONLY for now
#
if O_TRUNC in flags:
stuff
if O_RDRW in flags:
stuff
Which is pretty straight-forward. Circling back to O_RDONLY -- in the above
snippet we either:
- need to remember that that particular flag has an actual value of 0 (and one
of the big motivating
factors behind Enum was not having to worry about the actual value) and use a
different operator:
if not flags:
if flags == 0: # only works because OpenFlag is an IntFlag for backwards
compatibility
if flags.value == 0 # works for both Flag and IntFlag
or
- recognize that a nothing cannot be in a something:
# flags is O_RDONLY
if O_RDONLY in flags: # true
pass
# flags is O_RDRW
if O_RDONLY in flags: # false
pass
# flags is O_RDRW | O_TRUNC
if O_RDONLY in flags: # false
pass
--
~Ethan~
[1] https://en.wikipedia.org/wiki/Vacuous_truth -- last line of first paragraph
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