I'm not a Python dev, but I've been meaning to learn for a while. So this is really interesting stuff. A few questions, if you don't mind.
I understand mutability and immutability in other languages (and I gave your link a quick read to make sure there weren't any weird Python-specific rules), so I understand how the list can change and still be the same object, but a tuple or string would not. But why does that mean that the default parameter object remains in existence throughout all calls, instead of being recreated each time it is called?
Is there a reason for this being the default behavior? It seems like the majority of the time you would want to use a default parameter, you'd want it to behave like your bug-free examples.
Think of the default parameter values as arguments to the initializer for the function object. If you passed a list into the constructor of a class, you wouldn't be surprised that if you modified the list outside the class that it would modify the same list inside the class.
While that explains how it works, I actually completely agree with you. This is surprising behavior and, in a language that prides itself on not being surprising, seems, well, surprising.
I have to wonder if performance isn't the big reason for it. If your default is [], it isn't a big deal to re-evaluate, but if your default is get_default_cities_from_slow_web_service(), having that re-evaluated on every function call would be catastrophic. Given the choice between two negatives, the choice they made is probably reasonable.
Before I ever ask this question (I do a lot of tech interviews sadly) I always ask the candidate about object mutability vs immutability. Almost everyone knows the textbook answer, and only a few know the actual implications of it. This tests which they know :)
Default kwargs of a function are defined at function definition. However, they are only in scope, for the scope of said function. It is a weird but important subtle difference.
I understand mutability and immutability in other languages (and I gave your link a quick read to make sure there weren't any weird Python-specific rules), so I understand how the list can change and still be the same object, but a tuple or string would not. But why does that mean that the default parameter object remains in existence throughout all calls, instead of being recreated each time it is called?
Is there a reason for this being the default behavior? It seems like the majority of the time you would want to use a default parameter, you'd want it to behave like your bug-free examples.