[Python-ideas] Give ipaddresses an __index__ method

[Chris Angelico]

On Thu, Feb 15, 2018 at 11:18 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> This idea is inspired by Eric Osborne's post "Extending __format__
> method in ipaddress", but I wanted to avoid derailing that thread.
>
> I notice what seems to be an inconsistency in the ipaddress objects:
>
> py> v4 = ipaddress.IPv4Address('1.2.3.4')
> py> bin(v4)
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> TypeError: 'IPv4Address' object cannot be interpreted as an integer
>
> But that's surely not right: we just need to explicitly do so:
>
> py> bin(int(v4))
> '0b1000000100000001100000100'
>
> IP addresses are, in a strong sense, integers: either 32 or 128 bits.
> And they can be explicitly converted losslessly to and from integers:
>

Except that this computer's IPv4 is not 3232235539, and I never want
to enter it that way. I enter it as 192.168.0.19 - as four separate
integers. The __index__ method means "this thing really is an integer,
and can be used as an index". With IPv6, similar: you think about them
as eight separate blocks of digits.

IP addresses can be losslessly converted to and from strings, too, and
that's a lot more useful. But they still don't have string methods,
because they're not strings.

> I acknowledge one potentially undesirable side-effect: this would
> allow using IP addresses as indexes into sequences:
>
> py> 'abcdef'[X('0.0.0.2')]
> 'c'
>
> but while it's weird to do this, I don't think it's logically wrong.

That's not a side effect. That is the *primary* effect of __index__.
If you call int() on something, you are *converting* it to an integer
(eg int(2.3) ==> 2), and IMO that is the appropriate way to turn
192.168.0.19 into 3232235539 if ever you want that.

Unless you have a use-case for using IP addresses as integers,
distinct from Eric's ideas?

ChrisA