3 ms·

The annoyance of dataclasses, of course, is that they interact very awkwardly with immutability, which much of the Python ecosystem mandates (due to lacking val

• the__alchemist 2y ago
  Valid. One of my biggest (Perhaps my #1) fault with python is sloppy mutability and pass-by-value/reference rules.
  • Spivak 2y ago
    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.
    • o11c 2y ago
      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
      • cwalv 2y ago
        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
        • o11c 2y ago
          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.
          • ericvsmith 2y ago
            This is incorrect. The returned integer is a regular Python object, not some "unboxed" integer value.
      • ericvsmith 2y ago
        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.
• nomel 2y ago
  > is that they interact very awkwardly with immutability How so? Tuples are awkward with immutability, if you put mutable things inside them.
  • o11c 2y ago
    By default, dataclasses can't be used as keys in a `dict`. You have to either use `frozen` (in which case the generated `__init__` becomes an abomination) or use `unsafe_hash` (in which case you have no guardrails). In languages with value semantics, nothing about this problem even makes sense, since obviously a dict's key is taken by value if it needs to be stored. -- Tuple behavior is sensible if you are familiar with discussion of reference semantics (though not as much as if you also support value semantics). Still, at least we aren't Javascript where the question of using composite keys is answered with "screw you".
    • porridgeraisin 2y ago
      Haha obj[JSON.stringify(x)] = y * Ducks *
      • o11c 2y ago
        x = [Symbol('geese')]