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

Guido van Rossum guido at python.org
Wed Mar 14 19:32:49 CET 2012

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.


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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