[Python-ideas] set.add could return True or False

[Matt Joiner]

There's not much chance of that now, most projects that could  cause
trouble with this are using six, or moving up from 2. By comparison many
other things in Python 3.3 are very non backwards friendly like delegating
generators, new exception hierarchy, and a crap load of new API calls. Now
is the hour! :>
On Mar 15, 2012 2:33 AM, "Guido van Rossum" <guido at python.org> wrote:

> I think it would actually be reasonable to return a true/false value
> here (there are plenty of non-threaded uses for this), except I think
> it's too late to change now -- it would just encourage folks from
> writing code that works with Python 3.3 but is very subtly broken with
> earlier versions. So, -1.
>
> --Guido
>
> On Wed, Mar 14, 2012 at 11:29 AM, Matt Joiner <anacrolix at gmail.com> wrote:
> > I should not have emphasized the atomicity here, this was not intended
> to be
> > the main reason.
> >
> > On Mar 15, 2012 2:11 AM, "Chris Rebert" <pyideas at rebertia.com> wrote:
> >>
> >> On Wed, Mar 14, 2012 at 10:53 AM, Masklinn <masklinn at masklinn.net>
> wrote:
> >> > On 2012-03-14, at 18:36 , Matt Joiner wrote:
> >> >> set.add(x) could return True if x was added to the set, and False if
> x
> >> >> was already in the set.
> >> >
> >> > That does not mesh with the usual Python semantics of methods either
> >> > having a side-effect (mutation) or returning a value. Why would that
> >> > happen with sets but not with e.g. dicts?
> >>
> >> The rule is a bit more complicated than that (e.g. consider
> >> list.pop()). It's gets fleshed out well in:
> >> http://bugs.python.org/issue12192
> >>
> >> set.remove() arguably "returns" the same sort of indication as that
> >> which is proposed, in that it either raises or doesn't raise KeyError
> >> depending on whether the value was present.
> >>
> >> But yeah, these boolean return values aren't of huge utility,
> >> particularly in the multithreaded case.
> >>
> >> Cheers,
> >> Chris
> >
> >
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> >
>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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