[Python-Dev] PEP 351, the freeze protocol

Nick Coghlan ncoghlan at gmail.com
Tue Oct 25 11:26:58 CEST 2005

Josiah Carlson wrote:
> Nick Coghlan <ncoghlan at gmail.com> wrote:
>> I think having dicts and sets automatically invoke freeze would be a mistake, 
>> because at least one of the following two cases would behave unexpectedly:
> I'm pretty sure that the PEP was only aslomg if one would freeze the
> contents of dicts IF the dict was being frozen.
> That is, which of the following should be the case:
>     freeze({1:[2,3,4]}) -> {1:[2,3,4]}
>     freeze({1:[2,3,4]}) -> xdict(1=(2,3,4))

I believe the choices you intended are:
      freeze({1:[2,3,4]}) -> imdict(1=[2,3,4])
      freeze({1:[2,3,4]}) -> imdict(1=(2,3,4))

Regardless, that question makes a lot more sense (and looking at the PEP 
again, I realised I simply read it wrong the first time).

For containers where equality depends on the contents of the container (i.e., 
all the builtin ones), I don't see how it is possible to implement a sensible 
hash function without freezing the contents as well - otherwise your immutable 
isn't particularly immutable.

Consider what would happen if list "__freeze__" simply returned a tuple 
version of itself - you have a __freeze__ method which returns a potentially 
unhashable object!


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia

More information about the Python-Dev mailing list