Python's concept of immutability is useful, but it could be more general.
In the beginning, strings, tuples, and numbers were immutable, and everything else was mutable. That was simple enough. But over time, Python has acquired more immutable types - immutable sets and immutable byte arrays. Each of these is a special case. Python doesn't have immutable objects as a general concept, but it may be headed in that direction. There was some fooling around with an "immmutability API" associated with NumPy back in 2007, but that was removed. As more immutable types are added, a more general approach may be useful. Suppose, for discussion purposes, we had general "immutable objects". Objects inherited from "immutableobject" instead of "object" would be unchangeable once "__init__" had returned. Where does this take us? Immutability is interesting for threaded programs, because immutable objects can be shared without risk. Consider a programming model where objects shared between threads must be either immutable or "synchronized" in the sense that Java uses the term. Such programs are free of most race conditions, without much programmer effort to make them so. Java "synchronized" turned out to be a headache partly because trying to figure out how to lock all the "little stuff" being passed around a headache. But Java doesn't have immutable objects. Python does, and that can be exploited to make thread-based programming cleaner. The general idea is that synchronized objects would have built in locks which would lock at entry to any function of the object and unlock at exit. The lock would also unlock at explicit waits. A "Queue" object would be a good example of a synchronized object. With this mechanism, multi-thread programs with shared data structures can be written with little or no explicit locking by the programmer. If the restrictions are made a bit stricter, strict enough that threads cannot share mutable unsynchronized data, removal of the "global interpreter lock" is potentially possible. This is a route to improved performance on modern multi-core CPUs. John Nagle -- http://mail.python.org/mailman/listinfo/python-list