Mutable types are hiding everywhere — even in functions In Python, you can define functions with optional arguments — that is, arguments that you don’t need to state explicitly afterwards ...

Python’s development team chose defaults that reflect the most common real-world scenarios. But if you do need to tweak the way garbage collection works, you can use Python’s gc module.

Not sure why there's a hangup about immutable vs. mutable as Python doesn't distinguish them when passing references to them. For the record, this pseudocode demonstrates pass-by-reference.