> I would never expect, at first, numbers[0] to print 999 after these statements, etc.
You are not alone! I've always disagreed with the idea that imperative programming is "more intuitive", more familiar to many perhaps. In a functional programming language, modifications to lists/collections result in new copies (with maximal sharing) and do not mutate the original. Such "persistent" collections have to be designed and implemented to make these operations cheap. Copying an array is typically not cheap and so Python just creates a new reference (pointer) for my_numbers.
You are not alone! I've always disagreed with the idea that imperative programming is "more intuitive", more familiar to many perhaps. In a functional programming language, modifications to lists/collections result in new copies (with maximal sharing) and do not mutate the original. Such "persistent" collections have to be designed and implemented to make these operations cheap. Copying an array is typically not cheap and so Python just creates a new reference (pointer) for my_numbers.