On Sat, Jun 27, 2020 at 10:06 PM Rob Cliffe <rob.cli...@btinternet.com> wrote:
>
>
>
> On 27/06/2020 05:33, Chris Angelico wrote:
> > On Sat, Jun 27, 2020 at 2:29 PM Steven D'Aprano <st...@pearwood.info> wrote:
> >> Seriously, I genuinely thought that the existing behaviour was the
> >> opposite and that `add` unconditionally added the element. "Last seen
> >> wins". If I was designing sets, that's probably how I would design it.
> >> After all, it's called *add*, not *add if not already there*. I was so
> >> sure that this was the current behaviour that I didn't bother to check
> >> it before posting, which is rare for me.
> >>
> >> So I think this counts as the principle of maximal surprise :-)
> > Well, that's not how Python's dicts OR sets work, so that would be
> > inconsistent. But feel free to do it your own way in your own
> > language.
> >
> > ChrisA
> > _______________________________________________
> >
> I don't know what you mean Chris, ISTM that *is* the way dicts work:
>
> >>> d = { 'one' : 1 }
> >>> d['one'] = 1.0
> >>> d
> {'one': 1.0}
> >>>
> >>> d = { 'one' : 1 }
> >>> d.update( {'one' : 1.0} )
> >>> d
> {'one': 1.0}
>
> Have I misunderstood you?
Sets behave like dict keys, not their values. Obviously the last
*value* will win (because that's how assignment works), but with the
keys, the one already in the dictionary will be retained. Switching
your example around:
>>> d = {1: 'one'}
>>> d[1.0] = 'one'
>>> d
{1: 'one'}
>>> d = {1: 'one'}
>>> d.update({1.0: 'one'})
>>> d
{1: 'one'}
>>> dict(((1, 'one'), (1.0, 'one')))
{1: 'one'}
>>> {1: 'one', 1.0: 'one'}
{1: 'one'}
When sets were introduced, I believe this behaviour of dicts was
deliberately mimicked for sets, although those who've been Pythonning
for longer than I have would be better able to say.
ChrisA
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