[Python-Dev] Dicts are broken Was: unicode hell/mixing str and unicode asdictionarykeys

[Terry Reedy]

"Michael Hudson" <mwh at python.net> wrote in message 
news:2m3bccwopj.fsf at starship.python.net...
> Michael Chermside <mcherm at mcherm.com> writes:
>
>> I'm changing the subject line because I want to convince everyone that
>> the problem being discussed in the "unicode hell" thread has nothing
>> to do with unicode and strings. It's all about dicts.
>
> I'd say it's more to do with __eq__.  It's a strange __eq__ method
> that raises an Exception, IMHO.

I agree; a == b should always work, certainly unless explicitly programmed 
otherwise in Python for a particular class.  So I think the proper solution 
is fix the buggy __eq__ method to return False instead.  If a byte string 
does not have a default (ascii) text interpretation, then it obviously is 
not equal to any particular unicode text.

The fundamental axiom of sets and hence of dict keys is that any 
object/value either is or is not a member (at any given time for 'mutable' 
set collections).  This requires that testing an object for possible 
membership by equality give a clean True or False answer.

> Please do realize that the motivation for this change was hours and
> hours of tortous debugging caused by a buggy __eq__ method making keys
> "impossibly" seem to not be in dictionaries.

So why not fix the buggy __eq__ method?

>> 2.4, fails in 2.5, and arguably ought to work fine. I think we should
>> restore the behavior of dicts that when they compare keys for
>> equality they suppress exceptions (treating the objects as unequal),
>> or at LEAST retain the behavior for one more release making it a
>> warning this time.
>
> Please no.  Here's just one piece of evidence that the 2.4 semantics
> are pretty silly too:

We mostly agreed half a decode ago that 1/2 should be .5 instead of 0, but 
to avoid breaking code, have (or Guido has) refrained from yet making the 
change the default.  To me, the same principle applies here at least as 
strongly.

Terry Jan Reedy