Bug: Do not **unintentionally** modify function arguments

[igalic]

We have a handful of functions that take non-None arguments, and then modify them.
This leads to python directly modifying the passed argument, and is generally considered bad form.

One such example is JailConfigAddresses#add()

    def add(self, nic, addresses=[], notify=True):

        if isinstance(addresses, str):
            addresses = [addresses]

maybe there are linter params that will warn us of such code, until then we might need human eyes.

[gronke]

Agreed. Needs to be addressed! Should not be too complicated to fix, but a bit hard to spot. Is there a static code analyzer that does check for it?

[igalic]

i don't know…
it doesn't seem to be in pylint, https://pylint.readthedocs.io/en/latest/technical_reference/extensions.html
and we already use flake8, which dosen't seem to catch those either: http://flake8.pycqa.org/en/latest/user/error-codes.html

---

we should definitely (also?) consider using MyPy.
This one will also be able to check our type hints (#39).

[igalic]

after installing flake8-mutable, i get the following:

./iocage/lib/Network.py:36:33: M511 - mutable default arg of type List
./iocage/lib/DevfsRules.py:33:47: M511 - mutable default arg of type List
./iocage/lib/Jail.py:186:63: M511 - mutable default arg of type Dict
./iocage/lib/Filter.py:46:36: M511 - mutable default arg of type Call
./iocage/lib/NetworkInterface.py:34:33: M511 - mutable default arg of type List
./iocage/lib/NetworkInterface.py:35:33: M511 - mutable default arg of type List
./iocage/lib/NetworkInterface.py:43:33: M511 - mutable default arg of type List