[Python-ideas] Give ipaddresses an __index__ method

[Nick Coghlan]

On 15 February 2018 at 10:18, 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

That error message should probably either have an "implicitly" in it,
or else use the word "handled" rather than "interpreted".

There are tests that ensure IP addresses don't implement __index__,
and the pragmatic reason for that is the downside you mentioned: to
ensure they can't be used as indices, slice endpoints, or range
endpoints. While IP addresses can be converted to an integer, they are
*not* integers in any mathematical sense, and it doesn't make sense to
treat them that way.

A useful heuristic for answering the question "Should this type
implement __index__?" is "Does this type conform to the
numbers.Integral ABC?" (IP addresses definitely don't, as there's no
concept of addition, subtraction, multiplication, division, etc -
they're discrete entities with a numeric representation, not numbers)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia