On Mon, Nov 8, 2021 at 7:26 PM Ethan Furman <et...@stoneleaf.us> wrote:
> Let's use a concrete example: `re.RegexFlag`
>
> ```
> Help on function match in module re:
>
> match(pattern, string, flags=0)
> Try to apply the pattern at the start of the string, returning
> a Match object, or None if no match was found.
> ```
>
> In use we have:
>
> result = re.match('present', 'who has a presence here?',
> re.IGNORECASE|re.DOTALL)
>
> Inside `re.match` we have `flags`, but we don't know if `flags` is nothing
> (0), a single flag (re.ASCII, maybe) or a
> group of flags (such as in the example). For me, the obvious way to check
> is with:
>
> if re.IGNORECASE in flags: # could be re.I in 0, re.I in 2, re.I in
> 5, etc.
> ...
>
Yes.
Now, suppose for the sake of argument that there was a combination of flags
> that had its own code path, say
>
> weird_case = re.ASCII | re.LOCALE | re.MULTILINE
>
> I can see that going two ways:
>
> weird_case in flags # if other flags allowed
>
I would spell this as
if flags & weird_case == weird_case:
(which BTW works too if `flags` and `weird_case` are sets!)
or
>
> weird_case is flags # if no other flags allowed
>
this should be spelled using `==`, not using `is` (which is reserved for
identity, but here we're comparing values). And again this works for sets
too. (Just don't confuse set elements with subsets!)
> The idiom that I'm shooting for is using `in` to check for flags:
>
> - flag1 in flag1 True
> - flag1 in (flag1 | flag2) True
> - flag3 in (flag1 | flag2) True
>
Did you mean False for this last one?
- flag3 in flag1 False
> - flag3 in flag4 False
>
When `flag` is a single flag bit and `flags` is an unknown combination of
flag bits, you can conceivably make it so you can write this as `flag in
flags`. But `in` doesn't work when sets are represented as bits in an
integer (which is the analogy you're going for?)
> and
>
> - flag0 in any flag False
>
- any flag in flag0 False
>
What is `any flag` here?
> And, of course, if we want to know if the thing we have is exactly flag1:
>
> flag is flag1
>
> will tell us.
>
Please use `==` here.
> Does this make sense?
>
WHen thinking about sets of flag bits, do you actually visualize them as
bits in an integer? That's how I always think of them. Suppose we have
three flags, F1, F2 and F3. Let's write them in binary:
F1 = 0b001
F2 = 0b010
F3 = 0b100
Let's have a flags variable composed of F1 and F3:
flags = 0b101
F1 & flags is a bitwise AND of 0b001 and 0b101, which of course is 0b001
(and so on):
F1 & flags == 0b001 & 0b101 == 0b001
F2 & flags == 0b010 & 0b101 == 0b000
F3 & flags == 0b100 & 0b101 == 0b100
IF weird_case is F2 combined with F3, we get
weird_case == F2|F3 == 0b010|0b100 == 0b110
Etc., etc.
--
--Guido van Rossum (python.org/~guido)
*Pronouns: he/him **(why is my pronoun here?)*
<http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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