On 1 March 2012 12:08, Victor Stinner <victor.stin...@gmail.com> wrote: > New try: > > "A frozendict is a read-only mapping: a key cannot be added nor > removed, and a key is always mapped to the same value. However, > frozendict values can be mutable (not hashable). A frozendict is > hashable and so immutable if and only if all values are hashable > (immutable)."
I'd suggest you don't link immutability and non-hashability so tightly. Misbehaved objects can be mutable but hashable: >>> class A: ... def __init__(self,a): ... self.a = a ... def __hash__(self): ... return 12 ... >>> a = A(1) >>> hash(a) 12 >>> a.a=19 >>> hash(a) 12 Just avoid using the term "immutable" at all: "A frozendict is a read-only mapping: a key cannot be added nor removed, and a key is always mapped to the same value. However, frozendict values can be mutable. A frozendict is hashable if and only if all values are hashable." I realise this is a weaker statement than you'd like to give (immutability seems to be what people *really* think they want when they talk about frozen objects), but don't promise immutability if that's not what you're providing. More specifically, I'd hate to think that someone for whom security is an issue would see your original description and think they could use a frozendict and get safety, only to find their system breached because of a class like A above. The same could happen to people who want to handle thread safety via immutable objects, who could also end up with errors if misbehaving classes found their way into an application. Paul. _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
