[Python-ideas] Hexadecimal floating literals

[Chris Angelico]

On Tue, Sep 12, 2017 at 9:20 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Mon, Sep 11, 2017 at 06:26:16PM -0600, Neil Schemenauer wrote:
>> On 2017-09-12, Victor Stinner wrote:
>> > Instead of modifying the Python grammar, the alternative is to enhance
>> > float(str) to support it:
>> >
>> > k = float("0x1.2492492492492p-3") # 1/7
>>
>> Making it a different function from float() would avoid backwards
>> compatibility issues. I.e. float() no longer returns errors on some
>> inputs.
>
> I don't think many people will care about backwards compatibility of
> errors. Intentionally calling float() in order to get an exception is
> not very common (apart from test suites). Its easier to use raise if
> you want a ValueError.
>
> The only counter-example I can think of is beginner programmers who
> write something like:
>
> num = float(input("Enter a number:"))
>
> and are surprised when the "invalid" response "0x1.Fp2" is accepted. But
> then they've already got the same so-called problem with int accepting
> "invalid" strings like "0xDEADBEEF". So I stress that this is a problem
> in theory, not in practice.

Your specific example doesn't work as int() won't accept that by
default - you have to explicitly say "base=0" to make that acceptable.
But we have other examples where what used to be an error is now
acceptable:

Python 3.5.3 (default, Jan 19 2017, 14:11:04)
[GCC 6.3.0 20170118] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> int("1_234_567")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '1_234_567'

Python 3.7.0a0 (heads/master:cb76029b47, Aug 30 2017, 23:43:41)
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> int("1_234_567")
1234567

Maybe hex floats should be acceptable only with float(str, base=0)?

ChrisA