Avoid using a mutable default value as an argument

[didibz]

The purpose of this change is to avoid using a mutable default value as an argument, since it's not good to do so in python.
This is an article that demonstrate the issue:
https://docs.quantifiedcode.com/python-anti-patterns/correctness/mutable_default_value_as_argument.html

[jondot]

Thanks Didi
I would accept this if the parameters were actually mutated -- are they?
If so it could create interesting bugs

Otherwise what you're describing says -- what's the point in default params in Python, this feature is completely broken so let's never use it.

[didibz]

@jondot
In Python the default arguments are evaluated once when the function is defined, not each time the function is called. You should never define default arguments of mutable type unless you know what you are doing
The full article:
pythontips - Mutation

[didibz]

@jondot I agree that currently the arguments are not actually mutated. However, I see this more as good practice that removes a "trap" that may cause annoying bugs from future code changes that will mutate one of the arguments.

I think you can find the pattern on getting None instead of {}/[] as a default value in all popular Python open source libs (one example).

[didibz]

One last claim :) Is many cases you may pass the argument to another class/function/3rd-party (like here) that may mutate it. BTW, you can see that also CircuitBreaker uses this pattern.