On Mon, 29 Oct 2012 15:45:59 -0700, Chris Kaynor wrote: > On Mon, Oct 29, 2012 at 3:30 PM, Steven D'Aprano > <steve+comp.lang.pyt...@pearwood.info> wrote: >> On Mon, 29 Oct 2012 17:05:07 +0000, andrea crotti wrote: >> >>> I meant how do I create new immutables classes myself, I guess that's >>> possible writing C extensions but I don't see in pure Python.. >> >> Well, you can't *quite* make a truly immutable class in pure-Python, >> because if *your* Python code can manipulate the class during >> construction then so can the caller's Python code after construction. >> >> The trivial way to make an immutable class in Python is to inherit from >> an already immutable class and add behaviour but no state: >> >> class MyInt(int): >> def inc(self): >> return self.__class__(self + 1) >> >> >> Otherwise, you can add private state and rely on the caller not >> shooting themselves in the foot by accessing single-underscore names, >> use properties to protect private state, etc. >> >> > You'd also need to add __slots__ = () to the class definition to make it > immutable. Otherwise they still can shoot themselves in the foot by > adding new attributes.
"Doctor, it hurts when I do this." "Then don't do that." I'm not a big fan of preventatively using __slots__ merely to prevent the caller from tagging an object with extra data. Why do you care if the caller sticks a postit note on the object? It doesn't hurt the object, and if the caller loses track of which object has a postit note, that's their responsibility, not yours. I often wish I could sick an attribute on built-ins, e.g. after calculating some numeric result as a float, stick an error estimate on it. Callers who care about the error estimate can inspect it; those who don't, will never even notice it. If you have a good reason for using __slots__, then go right ahead. Otherwise, don't be paternalistic. This is Python, we have the right to shoot ourselves in the foot if we like. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
