Issue1622207

Jython seems to impose a non standard restriction here, as CPython doesn't have any problem with this code:

>>> import _weakref, UserDict
>>> _weakref.ref(UserDict.UserDict())

But it raises TypeError on Jython (2.3 branch at least).

I'm attaching a simple patch, but I don't know if that's the "right way" or not.

ugh, it almost seems like a bug to me that something so basic as hashCode is throwing Exceptions.  The patch is probably going to cover up something else that's more maniacal.

The attached patch solved my problem with weakref. My repro case is below.

from weakref import *
wvd = WeakValueDictionary()
try:
    wvd_hash = hash(wvd)
except Exception,e:
    print e, "this was expected"

try:
    wvd_ref = ref(wvd)
    print wvd_ref
except Exception, e:
    print e, "this exception was NOT expected, running under jython"



foo@foo-desktop:~/jython_dev$ python weakref_bug.py 
unhashable instance this was expected
<weakref at 0xb7dbdfcc; to 'instance' at 0xb7dbbcec>
foo@foo-desktop:~/jython_dev$ jython weakref_bug.py 
unhashable instance this was expected
<weakref 1; to 'instance' object>

If we could get any comments from a core jython dev on this bug that would be great.

The patch makes the hash value for all unhashable PyObjects 0.  Combined with the way GlobalRefs are used in a hashtable, this means that creating refs for two equal but distinct objects will only make a single ref.  This is undesirable as illustrated in the example I'm attaching: calling the second ref returns the first object.  

The only solution I can think of is to use System.identityHashCode(object) instead of object.hashCode().  As long as there isn't a case where Jython creates two separate Objects that refer to the same underlying Python object that should be fine.  I can't think of anything like that off the top of my head.
File Added: surprising_assignment.py

Even if the object defines a __hash__ method, the WeakValueDictionary does not work:

>>> class X(object):
       def __init__(self, id):
           self.id = id
       def __hash__(self):
           return hash(self.id)

>>> # Code taken from the CPython example:
>>> import weakref
>>> _id2obj_dict = weakref.WeakValueDictionary()
>>> def remember(obj):
       oid = id(obj)
       _id2obj_dict[oid] = obj
       return oid
>>> x = X(123)
>>> remember(x)
Traceback (innermost last):
...
TypeError: unhashable instance

This is fixed in r3259.