[Python-ideas] Fwd: Re: Unambiguous repr for recursive objects

[Wes Turner]

On Dec 26, 2015 4:05 PM, "Andrew Barnert" <abarnert at yahoo.com> wrote:
>
> On Saturday, December 26, 2015 8:48 AM, Wes Turner <wes.turner at gmail.com>
wrote:
>
>
> >So,
>
> >Would I be correct in that, because of this regression in __repr__
behavior (elipsis instead of total information) from 2.x to 3.x, any tests
that string compare __repr__ are now off?
>
> No. 2.x did not provide total information. It used the exact same
__repr__ as 3.x. (If you can come up with a way to provide total
information that's readable to both humans and the parser, I'm sure
everyone would love to see it.)
>
> Any tests that string compare __repr__ to test the equality of two lists
in 2.x will get the same results in 3.x. They're still probably bad tests,
but no worse than before. The only difference is that the `...` is a valid
literal in 3.x, so `[1, 2, [...]]` is a valid list display in 3.x, and it
wasn't in 2.x. (Even there, tests that assume __repr__ equality are no more
broken than before: a list containing 1, 2, and itself reprs as `[1, 2,
[...]]`, while a list containing 1, 2, and a list containing `...` reprs as
`[1, 2, [Ellipsis]]`, so they will not be mistakenly compared equal.)

Got it, thanks!

* https://docs.python.org/3/library/constants.html#Ellipsis
*
http://python-reference.readthedocs.org/ewhat-does-the-python-ellipsis-object-do
*
http://stackoverflow.com/questions/772124/what-does-the-python-ellipsis-object-do

>
> As Serhiy points out, there actually _is_ a regression here: tests that
depend on the fact that a circular list will raise a SyntaxError on
eval(repr(x)) do break with 3.0. I doubt there were many such tests, given
that nobody's noticed the problem until half a decade later, but I suppose
that is a regression.
>
> At any rate, I think what people are actually worried about here is not
the theoretical chance that such a regression might have happened 5 years
ago, but the more practical fact that 3.x might be misleading to human
beings in cases where 2.x wasn't. For example, if you mostly do 3.x NumPy
stuff, you're used to passing ellipses around, and maybe even storing them
in index arrays, but you rarely if ever see a circular list. So, when you
see, say, `[[1], [2], [...]]` on the REPL, you may misinterpret it as
meaning something different from what it does.
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