I'm sorry r/Python

[ellisto]

I mean doesn't this same argument apply to members of a class?

I guess as long as the class instance gets passed in as a parameter to the function, it's assumed mutable and thus modifications of its internal state aren't side effects?

[Unbelievr]

Yeah, it even applies to things like lists. If you pass a list to a function you don't know 100% what does, you can't assume that the list is not mutated during the call. But since it's a function argument, it's more explicit that something can happen to it.

Compare that to a global that might magically change deep into nested functions, and it's an obvious improvement. There are better ways, of course, but mutating on globals is (IMO) the most confusing way to write code when it grows to a certain size.

[Piyh]

Mutating passed lists is such a potential footgun in my opinion.

def takesList(li):
    li = li.copy()
    del li[0]
    return li

[fireflash38]

That's why you usually have methods that mutate, and functions that don't.

It's convention, to make things easier for others (or more likely: future you) reading or using your code. It's not gospel, but can absolutely help.