atf_python/vnet: Make it possible to set the FIB of vnet interfaces

[zlei]

tests/atf_python/sys/net/vnet.py

146

The 0 is a perfect valid fib number. Why not supporting setting fib 0 ?

[ngie]

tests/atf_python/sys/net/vnet.py

60

Setting a default here would be wise: that allows you to skip a lot of the baked-in defaults elsewhere in the diff.

Other considerations:

• It might be a really good idea to create a setter to validate the input argument before setting the value as well. typing.assert_type could help enforce the type, unless you want to get fancy and implement an isinstance / ValueError pattern, but you should also ensure that the domain is >= 0, since negative FIB values don't make sense.
• It might be a good idea to use a constant here as well, e.g., NO_FIB.

146

No need for an explicit test, per PEP8.

381–384

General comment: using dict.get with hardcoded defaults sprinkled around instead of using explicit property variables adds a lot of fragile complexity in this module (and other API consumers):

1. If defaults change, you now have to do a sweeping change to update all of the defaults around the module and the consumers.
2. It makes it difficult to impossible for static analyzers like flake8 or ruff to figure out what's going on in the code.

[markj]

tests/atf_python/sys/net/vnet.py

146

Only because that's the default. But yes we could set it unconditionally. The only downside is an extra fork+exec per interface (which isn't free when running tests under QEMU).