[Tutor] mutability, __new__

[Terry Carroll]

[hijacking the "question about object oriented programming and 
inheritance using datetime module" thread here....]

On Tue, 16 Jan 2007, Kent Johnson wrote:

> I think you are missing the line
>    datetime.date.__init__(self, year, month, day)
> somewhere in here.

Yeah; sloppy of me.

> I am very surprised that this works, my understanding was that 
> datetime.date was immutable and required overriding __new__() rather 
> than __init__(). But it does work...

Yeah, come to think of it, that is interesting.

A quick test shows that datetime.date is apparently immutable, because it 
can be used as a dictionary key.

But if it's immutable, then __init__ shouldn't work, and, as you say, you 
gotta go with __new__.

I have to admit, I don't completely comprehend the use of __new__, but the 
discussion at http://www.python.org/download/releases/2.2.3/descrintro/ 
explained quite a lot.

How can you find if a particular object is immutable or not?  Or, 
relatedly, if you wanted to define an object that was immutable, how would 
you do it?