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

[Eric Snow]

On Thu, Oct 6, 2011 at 9:28 PM, Eric Snow <ericsnowcurrently at gmail.com> wrote:
> 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/
>

Okay, it's like Raymond was reading my mind in some of what he says here:

  http://mail.python.org/pipermail/python-dev/2006-February/060802.html

Now I'm gonna have to go back consider if there is a better way to do
the read-only bit I mentioned earlier (which motivated this post).  My
only hope is that he is talking about something else.  :)  Regardless,
I suppose the recipe will stand or fall on its own anyway.

-eric

p.s. Greg Ewing also makes a pretty good point:
http://mail.python.org/pipermail/python-dev/2006-February/060822.html