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Comment by the__alchemist | Hacker News Reader

Valid. One of my biggest (Perhaps my #1) fault with python is sloppy mutability and pass-by-value/reference rules.

Is there a situation where Python ever passes by value? Like you can sort of pretend for primitive types but I can't think of case where it's actually value.

Non-CPython implementations may pass by value as an optimization for certain immutable builtin types. This is visible to code using `is`.

(It's surprisingly difficult to implement a rigorous way to detect this vs compile-time constant evaluation though; note that identical objects of certain types are pooled already when generating/loading bytecode files. I don't think any current implementation is smart enough to optimize the following though)

  $ python3 -c 'o = object(); print(id(o) is id(o))'
  False
  $ pypy3 -c 'o = object(); print(id(o) is id(o))'
  True
  $ jython -c 'o = object(); print(id(o) is id(o))'
  True

Are you saying `o` is passed by value? I think this behavior is due to the return from `id()` being interned, or not. `id(o) == id(o)` will be true in all cases

I mean that the `id` function returns by value. It's not interning since that explicitly refers to something allocated, which isn't the case here.

This is incorrect. The returned integer is a regular Python object, not some "unboxed" integer value.

What you're seeing here is an optimization about integers, not about pass by value. CPython only does this for small integers:

  $ python -c "print(int('3') is int('3'))"
  True
  $ python -c "print(int('300') is int('300'))"
  False

Other implementations make different choices.

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