The ip_network class in the ipaddress module does cache the broadcast_address attribute.
But when the network address is changed the cache is not cleared/updated.

Example:

> from ipaddress import ip_network
> 
> print("------------------------------")
> 
> net = ip_network("10.0.0.0/8")
> print("net", net)
> print("broadcast_address", net.broadcast_address)
> print("increase")
> net.network_address += 2**24
> print("net", net)
> print("net.broadcast_address", net.broadcast_address)
>
> print("------------------------------")
>
> net2 = ip_network("10.0.0.0/8")
> print("net2", net2)
> print("no call to broadcast_address")
> print("increase")
> net2.network_address += 2**24
> print("net2", net2)
> print("net2.broadcast_address", net2.broadcast_address)

All classes in the ipaddress module are designed as immutable.

While it is not documented, you can see __hash__ is overridden.  It means you must not change the object state.

since it hasn't been documented that those classes are all designed to be immutable (and many things work when they are mutable) many people probably use it that way.
Declaring them immutable via the docs now would "break" existing code.

It is documented, if you read it carefully.

"Network objects are hashable, so they can be used as keys in dictionaries."

https://docs.python.org/3/library/ipaddress.html#network-objects

"""
An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value.
"""

https://docs.python.org/3/glossary.html#term-hashable

Make it mutable is not an option.

I close this issue as rejected.
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