On Fri, Jun 26, 2020 at 06:16:05AM -0500, Dan Sommers wrote:
> > already_there = seen.add(element)
> > if already_there:
> > # handle the duplicate case
> >
> >Who thinks like that? *wink*
>
> Anyone who practices EAFP rather than LBYL? Or is that why you're
> winking?
That doesn't come naturally, and in single-threaded code it's also
unnecessary.
By the way, since there's no try...except involved and no need to catch
an exception, it's not EAFP.
> >But either way, you also have to decide whether the `add` (or
> >the new
> >method) should *unconditionally* insert the element, or only do
> >so if it
> >wasn't present. This makes a big difference:
> >
> > seen = {2}
> > already_there = seen.add(2.0)
> >
> >At this point, is `seen` the set {2} or {2.0}? Justify why one
> >answer is
> >the best answer.
>
> The actual best answer is left as an exercise for the interested
> reader, but whatever it is, it's justified by backwards
> compatibility, the existing definition of "present," and the
> principle of least surprise:
The documentation doesn't guarantee one behaviour or another:
https://docs.python.org/3/library/stdtypes.html#frozenset.add
> Python 3.8.3 (default, May 17 2020, 18:15:42)
> [GCC 10.1.0] on linux
> Type "help", "copyright", "credits" or "license" for more
> information.
> >>> seen = {2}
> >>> 2.0 in seen
> True
> >>> seen.add(2.0)
> >>> seen
> {2}
Well I'm completely surprised, because I expected `add` to, you know,
actually add the element replacing the one that was already there!
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 :-)
Should the flag be "element was already present and nothing was added"
or "element was not there, so something was added"?
--
Steven
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