[Python-ideas] A Protocol for Making an Object Immutable

[Eric Snow]

As I was working on some code today, I found myself in a situation
where I wanted to "freeze" an object.  This topic has come up a few
times since I've been following these lists, so I figured it wasn't
too crazy an idea.

Really quickly I hacked out a simple bit of code that I am using for
what I needed, and later I posted it as a recipe on the ActiveState
cookbook[1].  The gist of it is a __freeze__/__unfreeze__ protocol,
supported respectively by two ABCs and two functions (freeze and
unfreeze).

I've come to learn that it's a good idea to do some digging in the
list archives before posting an idea, and I'm glad I did.  Apparently
there is a nearly identical PEP (351) to what I was thinking of [2].
And just my luck, it was shot down pretty hard.

So I have a couple questions.  First, from what I've read so far, the
idea of frozen/immutable seems to be tied pretty closely to being
hashable.  However, this wasn't applicable to the use case that I had.
 Instead, I just wanted to make sure certain attributes of a class of
mine were effectively read-only.  So, is being hashable necessarily
tied to being immutable?

Second, I realize that PEP 351 failed to pass muster, so I'm not
holding my breath here.  Shoot, I'm not even really proposing we add
this protocol.  Mostly, I found a simple model that worked for me and
seemed more broadly applicable; and I wanted to see if it would be
suitable for further research.  Now, in light of PEP 351, I'm less
convinced that it's so simple.  Still, I've seen some ideas come back
from the proverbial grave and wanted to see if this one is not quite
dead, sir (or if I should avenge it <wink>).

-eric


[1] http://code.activestate.com/recipes/577895/
[2] http://www.python.org/dev/peps/pep-0351/