Re: Migrating to BTrees 4.x

On 11/19/16 3:58 PM, Jim Fulton wrote:
>
>
> On Sat, Nov 19, 2016 at 3:21 PM, David Glick <[email protected] 
> <mailto:[email protected]>> wrote:
>
>     I'm working on updating Plone to support ZODB 4 and 5.
>
>     So far most of the blockers are because BTrees 4.x no longer
>     allows using keys whose ordering is not well-defined, including
>     None, and there are various places where Plone was using None as a
>     key. I understand the reason for the restriction and am making
>     progress on making Plone no longer do this.
>
>
> Hm. IMO, the restriction should be in adding items, not on 
> unpickling.  We should go to great lengths, to avoid unpickling errors.
>
> I think we should avoid raising this error in __setstate__. It should 
> be easy to fix.

I was hoping you might feel that way.

I started implementing the fix and am able to make it ignore the default 
comparison TypeError during __setstate__. However, even with the state 
loaded, `__getitem__(None)` and `__delitem__(None)` raise errors, 
because those operations are checking for keys with default comparison:

 >>> t = BTrees.OOBTree.OOBTree()
 >>> bucket_state = ((None, 42),)
 >>> tree_state = ((bucket_state,),)
 >>> t.__setstate__(tree_state)
 >>> t[None]
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
KeyError: None
 >>> del t[None]
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: Object has default comparison

So, still not clear how to remove the `None` key to fix the btree. Well, 
I guess I can at least construct a replacement BTree by filtering the 
items now:
 >>> list(t.items())
[(None, 42)]
 >>> cleantree = BTrees.OOBTree.OOBTree([(k, v) for k, v in t.items() if 
t is not None])

David

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