On Fri, Sep 04, 2020 at 09:40:55PM -0400, Cade Brown wrote:
> The `eval(repr(x)) == x` is not a segment of my code; rather it is part of
> Python's description of what 'repr' should do:
>
> https://docs.python.org/3.4/library/functions.html?highlight=repr#repr
>
>
> Specifically: ` For many types, this function makes an attempt to return a
> string that would yield an object with the same value when passed to eval()`
> <https://docs.python.org/3.4/library/functions.html?highlight=repr#eval>
"For many types" and "makes an attempt".
There has never been, and never will be, a guarantee that all objects
will obey that invariant. As I said, it is a Nice To Have, and it is
designed for convenience at the interactive interpreter.
> So everyone in this thread can stop mentioning security concerns; I'm sure
> we're all aware of those and we should instead focus on what repr should do
> and shouldn't do.
You specifically said that math.inf doesn't solve your problem *because*
`eval(repr(x))` doesn't work. Now you are backpeddling and saying that
this is not your actual problem.
(In fact it does work, if you do it correctly.)
There are a million other objects that don't obey that invariant:
py> x = object()
py> eval(repr(x)) == x
SyntaxError: invalid syntax
Why is float infinity so special that it needs to obey the
invariant?
What's the actual problem, or problems, in your code that you are trying
to solve by making an infinity builtin? If there is no actual problem
being solved, and the only reason you want this is because:
> I think it's weird to not fulfill this promise
you don't have any sympathy from me:
- `eval(repr(x))` is not a promise, it is a mere suggestion
that *some* types *try* to provide.
- Adding a special built-in constant Infinity just to satisfy
this Nice To Have feature is overkill.
- It would require float infinities to change their repr from
'inf' to 'Infinity', and that will break doctests.
- And even if that feature were satisfied by infinity, it
couldn't be satisfied by float NANs by their very definition:
py> from math import nan
py> nan == nan
False
So while the cost of adding a new Infinity builtin is small, the benefit
is even smaller.
--
Steve
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