[Zope-ZEO] Need for PersistentList

[John D. Heintz]

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Well, I thought about what you said and now I'm really confused.  The 
attached script is doing thing that I just don't understand.  It seems 
that Persistent object that implement a __hash__ method are indeed 
hashable, but dictionaries ignore the result of __hash__.  Also, it 
looks like the hashing is consitent across db connections.

John

Andrew Kuchling wrote:

> On Thu, Jan 18, 2001 at 04:35:37PM -0600, John D. Heintz wrote:
> 
>> Finally, we need for Persistent classes to be hashable.
> 
> 
> Hmm... Does __hash__() not work on an ExtensionClass?  Python will use
> the id() of an object, but that's obviously not applicable to a
> persistent object.  The OID can't be used either, since no OID is
> assigned until you do a commit.  I'd argue that, if a persistent class
> doesn't explicitly define __hash__, it shouldn't be hashable at all
> (in which case I've got some code that will break :) ).
> 
> --amk
> 
> 
> 
> _______________________________________________
> Bug reports, feature requests, etc. go in the ZEO Tracker:
> http://www.zope.org/Products/ZEO/Tracker
> 
> Conversations etc. can take place in the Wiki:
> http://www.zope.org/Products/ZEO/Wiki
> 
> Zope-ZEO maillist  -  Zope-ZEO@zope.org
> http://lists.zope.org/mailman/listinfo/zope-zeo



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John D. Heintz | Senior Engineer

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T 512.633.1198 | jheintz@isogen.com

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import Persistence
import ZODB
from PersistentMapping import PersistentMapping
from ZODB.POSException import ConflictError
import threading

from ZODB import DemoStorage
storage = DemoStorage.DemoStorage()

db = ZODB.DB(storage)

class P1(Persistence.Persistent):
    def __init__(self, name):
        self.name = name

class P2(Persistence.Persistent):
    def __init__(self, name):
        self.name = name
    def __hash__(self):
        return hash(None)

#Show that P1 is not hashable
try:
    {P1('a'):1}
except:
    print "P1 is not hashable."
else:
    print "P1 is hashable!!!!"


# Open initial connection and create a PersistentMapping
# with a two P2 instances
conn1 = db.open()
root1 = conn1.root()

p2A = P2('a')
print hash(p2A)
p2B = P2('b')
print hash(p2B)
root1['p2List'] = (p2A, p2B)

p2Map = PersistentMapping()
p2Map[p2A] = 'a'
p2Map[p2B] = 'b'
assert len(p2Map) == 2
root1['p2Map'] = p2Map
print p2Map._container
get_transaction().commit()

# Get second connection and retrieve maps

conn2 = db.open()
root2 = conn2.root()
    
p2List = root2['p2List']
p2Map = root2['p2Map']
print p2Map._container
    
for p2 in p2List:
    print hash(p2)
    assert p2.name == p2Map[p2]


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