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It's best practice to avoid mutable defaults even if you're not planning to mutate the argument.

It's just slightly annoying having to work around this by defaulting to None.

You don't need to use `None`. If you indeed aren't planning to mutate the argument, then use something immutable that provides the necessary interface. Typically, this will be `()`, and then your logic doesn't require the special case. I genuinely don't understand, after 20+ years of this, why everyone else has decided that the `None` check should be idiomatic. It's just, ugh. I'm pretty sure I've even seen people do this where a string is expected and `''` is right there staring at them as the obvious option.

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