Ethan Furman added the comment:
> I am quite aware about what's the intended use, but you can't just
> assume people will know about it.
Fair enough.
> In my view, you must do one of two things:
>
> 1) (at least in documentation, and preferably in the code by raising
> Exceptions at class definition time) forbid the use of Flags whose values are
> not either a) powers of two, or b) bitwise or of some already declared values
The closest I would get to forbidding something would be to forbid non-integers
as values -- and I'm not going to do that. In general, Python doesn't take
such steps (one notable exception being sum refusing to work with strings
because it was such a common trap).
> -or-
>
> 2) Specify and implement a robust algorithm for finding the "cover of
> bits" for a given numeric value. If the intended algorithm is really
> "pick the largest member value smaller than given value, subtract and
> repeat until 0 remains", then it should be said so in the
> documentation, and preferably some reasons given for the usage of that
> exact algorithm. ("it was easiest to implement" does not rank very high
> on my list.)
How about "it works for me"? Seriously, if every bit is named then the exact
repr used is irrelevant. If you don't like the standard method then write your
own __repr__ method, because that's what we're talking about -- not the actual
value (which isn't going to be affected by the algorithm for finding the "cover
of bits", but the display of the names).
> In other words, MyFlags(7) should either be illegal, or I should be
> able to interpret what it will be by reading the documentation. Leaving
> it unspecified is not acceptable IMO.
Thankfully, docs can still change during beta.
> (In case it isn't clear: I'm for option 1. I _don't_ intend to write
> MyFlags ever in Python. But if I happen to write it unintentionally
> (e.g. if I forget to declare 2), I would like Python to tell me I'm
> doing something wrong.)
We could certainly add a decorator that double-checks that, just like we have
the `unique` decorator to ensure no duplicates in an Enum. Any idea what to
call it? complete? named? allbits?
> (But if you really want option 2 for some reason, that's ok too. I'm
> just saying I would like it specified, with a rationale if it's not
> too much of a problem.)
(1) isn't going to happen, except possibly as a decorator. If you would like
to contribute code and/or docs for (2) and/or a decorator for (1) I'd be happy
to review.
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<http://bugs.python.org/issue23591>
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