[ZODB-Dev] Issue trying to remove elements in OOTreeSet

[Jim Fulton]

On Fri, Sep 24, 2010 at 5:03 PM, Marius Gedminas <marius at gedmin.as> wrote:
> On Fri, Sep 24, 2010 at 11:09:58AM -0400, Jim Fulton wrote:
>> I wasn't proposing to change anything. :) Breaking inheritance to avoid
>> inheriting a bad cmp seems rather too tricky.  (I assume there isn't
>> a default comparison in Python 3, although I haven't checked.)
>>
>> I suppose that the BTree implementation could check for object's
>> bad comparison operator and fail if it sees it. (Details omitted. :)
>>
>> This obviously wouldn't happen in 3.10, but might be a fun project for
>> someone in 3.11.
>
> What would you think of adding
>
>    def __lt__(self, other):
>        if not isinstance(other, Persistent):
>            raise ValueError('cannot compare Persistent with %s' % repr(other))
>        if self._p_oid is None or other._p_oid is None:
>            raise ValueError('cannot compare objects that have no persistent IDs yet; call connection.add()')
>        return self._p_oid < other._p_oid
>
>    # define __le__, __gt__, __ge__ in a similar way

Let's not forget __cmp__. :/

> to persistent.Persistent?

First, The above code is wrong. :)

- It doesn't dispatch to other, which might have custom comparison
  if its own.

- It doesn't take database name into account.

More importantly, your suggestion is tantamount to the same mistake
Guido made when he gave objects a default comparison operator.

If I was going to do anything, I would try to detect when a broken
default was used and fail.

BTW, I'm happy to see:

Python 3.1.2 (r312:79147, Sep  1 2010, 14:43:10)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class C: pass
...
>>> c = C()
>>> c < None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: C() < NoneType()
>>> C() < C()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: C() < C()

:)

Jim

--
Jim Fulton