On Sat, Jun 27, 2020 at 10:06 PM Rob Cliffe <rob.cliffe@btinternet.com> wrote:

On 27/06/2020 05:33, Chris Angelico wrote:

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On Sat, Jun 27, 2020 at 2:29 PM Steven D'Aprano <steve@pearwood.info> wrote:

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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:

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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:

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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