gh-153769: Clarify the TypeError from ipaddress subnet_of()/supernet_of()

[fedonman]

Calling subnet_of() or supernet_of() on a network and passing something that is not a network object, such as an address, gives a confusing error today:

>>> import ipaddress
>>> ipaddress.IPv6Network('::/0').subnet_of(ipaddress.IPv6Address('::1'))
AttributeError: 'IPv6Address' object has no attribute 'network_address'

During handling of the above exception, another exception occurred:

TypeError: Unable to test subnet containment between ::/0 and ::1

An address has a version attribute, so the version check passes, and the code then reaches for network_address, which the address does not have. The AttributeError is caught and re-raised as a TypeError whose message reads like a real containment result and hides what actually went wrong.

This checks that both arguments are network objects before touching any attributes, and raises a clear TypeError that says a network object is required. Nothing masks an internal AttributeError anymore:

>>> ipaddress.IPv6Network('::/0').subnet_of(ipaddress.IPv6Address('::1'))
TypeError: Unable to test subnet containment between ::/0 and ::1: both must be network objects

The existing behaviour for valid networks and for the mixed IPv4/IPv6 case is unchanged.

• Issue: ipaddress: subnet_of() and supernet_of() raise confusing TypeError wrapping an AttributeError when passed an Address object #153769

[python-cla-bot]

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