[zahlman]

There is no copy on write and no pass by reference.

Python is "pass by value", according to the original, pedantic sense of the term, but the values themselves have reference semantics (something that was apparently not contemplated by the people coming up with such terminology — even though Lisp also works that way). Every kind of object in Python is passed the same way. But a better term is "pass by assignment": passing a parameter to an argument works the same way as assigning a value to a variable. And the semantic distinctions you describe as nonexistent are in fact easy to demonstrate.

The model is easy to explain, and common in modern programming languages. It is the same as non-primitive types in Java (Java arrays also have these reference semantics, even for primitive element types, but they also have other oddities that arguably put them in a third category), or class instances (as opposed to struct instances, which have value semantics) in C# (although C# also allows both of these things to be passed by reference).

The pre-allocated integer objects are a performance optimization, nothing to do with semantics.

The model is useful for programming, because it's correct. We know that Python does not pass by reference because you cannot affect a caller's local variable, and thus cannot write a "swap" function. We know that Python copies the references around, rather than cloning objects, because you still can modify the object named by a caller's local variable. We know that no copy-on-write occurs because we can trivially set up examples that share objects (including common gotchas like https://stackoverflow.com/questions/240178).