On Sun, Jun 21, 2015 at 11:59 AM, Rustom Mody <rustompm...@gmail.com> wrote: > Recent thread on python ideas > https://mail.python.org/pipermail/python-ideas/2015-June/034177.html > > Since "python's immutable" ≠ "really immutable", we now need a "really > immutable"
That's because it requires mutable memory to keep track of reference counts. In a non-refcountinig Python, strings and integers can be truly immutable; that thread is suggesting (among other options) simply storing the refcounts in a separate table. Conceptually, a string or integer IS immutable, but CPython currently needs to be able to fiddle with some bookkeeping data about them, in order to pretend it has plenty of memory. A Python implementation could theoretically just abandon its strings whenever it's done with them, wasting gobs of memory in the process, and having them in read-only memory. More practically, a pure mark-and-sweep GC (such as MicroPython uses) trades prompt disposal for simple management, and in doing so, achieves (I think) the same total immutability. You have to distinguish between Python (the language) and CPython (the implementation) here. Python's notion of mutability has nothing to do with thread safety. You could achieve perfect thread safety for immutables by simply replicating them into every thread; it'd be inefficient, but perfectly legal. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
