ipaddress.IPv6Address.is_loopback broken for IPv4-mapped addresses

[paravoid]

Bug report

Bug description:

While properties like is_private account for IPv4-mapped IPv6 addresses, such as for example:

>>> ipaddress.ip_address("192.168.0.1").is_private
True
>>> ipaddress.ip_address("::ffff:192.168.0.1").is_private
True

...the same doesn't currently apply to the is_loopback property:

>>> ipaddress.ip_address("127.0.0.1").is_loopback
True
>>> ipaddress.ip_address("::ffff:127.0.0.1").is_loopback
False

At minimum, this inconsistency between different properties is counter-intuitive. Moreover, ::ffff:127.0.0.0/104 is for all intents and purposes a loopback address, and should be treated as such.

For the record, this will now match the behavior of other languages such as Rust, Go and .NET, cf. rust-lang/rust#69772.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-117566: fix IPv6Address.is_loopback for IPv4-mapped loopbacks #117567
• [3.12] gh-117566: fix IPv6Address.is_loopback for IPv4-mapped loopbacks (GH-117567) #118391

[paravoid]

This was fixed in main, as @encukou merged the PR. The only outstanding question I have is whether this warrants a backport to 3.12.x or not. Depending on the answer, this may be resolved :)

[encukou]

The answer is not obvious.
But given a similar PR is being backported, I say this one should be backported as well.

[paravoid]

@encukou thanks so much for the review, good feedback, merge & backport -- much appreciated! ❤️

[jstasiak]

Hey all, sorry for a late comment but I only now saw this issue randomly.

For the record, this will now match the behavior of other languages such as Rust, Go and .NET, cf. rust-lang/rust#69772.

Just to clarify, Rust's behavior is deliberately the opposite of this, as in it doesn't interpret IPv4-mapped IPv6 addresses like that[1][2].

I would argue that

Moreover, ::ffff:127.0.0.0/104 is for all intents and purposes a loopback address, and should be treated as such.

is not necessarily the only or even the most natural approach. For example with this patch there's no built-in way to check strictly for the IPv6 loopback address and nothing else.

And yes, I'd argue that is_private and is_global interpreting IPv4-mapped IPv6 addresses is IMO a mistake too and the decision is best left to the developer (the ipv4_mapped property can be used to convert if/when needed).

[1] rust-lang/rust#86335
[2] https://doc.rust-lang.org/std/net/struct.Ipv6Addr.html#ipv4-mapped-ipv6-addresses