On Sun, 20 Nov 2005 08:56:33 +1100, Ben Finney wrote: > Steven D'Aprano <[EMAIL PROTECTED]> wrote: >> On Fri, 18 Nov 2005 14:32:46 +1100, Ben Finney wrote: >> > Is there any difference between a Python immutable value, and a >> > constant? I suppose "constant" also implies that the *name* binds >> > unchangeably to a particular value. Is that meaningful? >> >> That's precisely how I understand "constant" to be useful. > > The Python docs don't really talk about such a thing. However, the > None name cannot be re-bound. Is that all that's required to have a > "constant" in Python?
I think None is a special case. The docs have said that None will become a keyword in the future. > If so, it's part of the assignment statement, and not something the > object itself can decide. True? Yes, that would be how I interpret constants: You want a name which can't be re-bound to something else. One work-around is to use the convention of writing the name in all caps: CONSTANT = some_value and then trust that your module user doesn't rebind CONSTANT. I'd like to see support for something a little stronger than just "cross your fingers and hope", without necessarily going all the way to Pascal's full enforcement of constants. "We're all adults here" -- if somebody *really* wants to rebind CONSTANT, I'm not going to stop them, but I want them to jump through a hoop to do it, as a reminder that the module creator thinks they shouldn't be doing it. The obvious analogy is with name mangling of __private attributes in classes. >> > How does one actually ensure an object is immutable? Is it a matter >> > of overriding a bunch of methods, or is ther a neater way? > > Really hoping someone can come up with an answer for this. So am I :-) Have you looked at the source for Frozenset? It was a pure Python module in Python 2.3, before becoming a built-in in 2.4. -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
