[Python-ideas] IntFlags

[Neil Girdhar]

On Sat, Mar 7, 2015 at 10:05 AM, Ethan Furman <ethan at stoneleaf.us> wrote:

> On 03/07/2015 05:53 AM, Andrew Barnert wrote:
> > On Mar 7, 2015, at 12:07 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> >>
> >>> On 03/03/2015 07:52 AM, Serhiy Storchaka wrote:
> >>>
> >>> We need new type IntFlags. It is like IntEnum, but has differences:
> >>>
> >>> 1. The value of an instance should be not limited to the set of
> predefined constants. It can be a combination of
> >>> predefined constants or even arbitrary integer.
> >>>
> >>> 2. The result of "|", "&" and "~" operators for IntFlags arguments
> should be an instance of the same IntFlags subclass.
> >>>
> >>> 3. It should have nice str() and repr().
> >>
> >> As long as we are dreaming  :)
> >>
> >> class Stat(IntFlag):
> >>    RDONLY = 1
> >>    NOSUID = 2
> >>    NODEV = 4
> >>    NOEXEC = 8
> >>    SYNCHRONOUS = 16
> >>    MANDLOCK = 64
> >>    WRITE = 128
> >>    APPEND = 256
> >>    NOATIME = 1024
> >>    NODIRATIME = 2048
> >>    RELATIME = 4096
> >>
> >> a = Stat.RDONLY  # creates a new instance of Stat, not a singleton
> >
> > Why?
>
> Because by having mutable instances of Stat we can have more Python
> operations:
>
> instead of:
>
>   x = Stat(some_Stat_value_from_somewhere)
>   x = x | Stat.NOEXEC  # to set the bit
>
> we can say:
>
>   x = Stat(some_Stat_value_from_somewhere)
>   x.NOEXEC = True
>
> or, even more illustratively, instead of:
>
>   x = Stat(some_Stat_value_from_somewhere)
>   x &= ~Stat.NOEXEC  # to clear the bit
>
> we can have:
>
>   x = Stat(some_Stat_value_from_somewhere)
>   x.NOEXEC = False  # much more readable
>

Yes, +1

>
>
> > Assuming the values are immutable (like int), [...]
>
> Drat.  Instances of Stat would not be immutuable, but that means they
> can't be thin wrappers around `int`, doesn't it?
> Which also means more work around C call sites.  Drat and double-drat.
>

Can't your C call sites cast the object to int?


>
>
> >> repr(~d)  #
> <Stat.RELATIME|NODIRATIME|NOATIME|512|APPEND|WRITE|SYNCHONOUS|NOEXEC|NODEV|NOSUID:
> 8094>
> >>
> >> I'm not at all sure I have that last one correct.
> >
> > That last one is the big question. In C, ~d is going to have the 8192,
> 16384, ... 2b
> > bits set, not just the bits you defined and the gaps in between. Are you
> sure you want
> > a different result in Python? (Especially if you're on a platform that
> does something
> > with those extra bits, so you can get values back that actually have
> them set.
> > Although I don't think that's an issue with stat, it is with lots of
> other flags from
> > POSIX-land.)
>
> A `bytes` (or `byte_size`) would need to be set to control how many bits
> got flipped.  If not set, then only defined
> bits get flipped.
>

Why do you need ~ at all?  Do any API calls that you want to make want the
inverted flags?  Isn't the only point of inverting the bits in order to
clear a field?


>
> --
> ~Ethan~
>
> --
>
> ---
> You received this message because you are subscribed to a topic in the
> Google Groups "python-ideas" group.
> To unsubscribe from this topic, visit
> https://groups.google.com/d/topic/python-ideas/L5KfCEXFaII/unsubscribe.
> To unsubscribe from this group and all its topics, send an email to
> python-ideas+unsubscribe at googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
> _______________________________________________
> Python-ideas mailing list
> Python-ideas at python.org
> https://mail.python.org/mailman/listinfo/python-ideas
> Code of Conduct: http://python.org/psf/codeofconduct/
>
> --
>
> ---
> You received this message because you are subscribed to a topic in the
> Google Groups "python-ideas" group.
> To unsubscribe from this topic, visit
> https://groups.google.com/d/topic/python-ideas/L5KfCEXFaII/unsubscribe.
> To unsubscribe from this group and all its topics, send an email to
> python-ideas+unsubscribe at googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150307/579993c1/attachment.html>