[Jython-bugs] [ jython-Bugs-1622207 ] _weakref.ref(o) only works if the argument is hasheable

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Bugs item #1622207, was opened at 2006-12-25 22:45
Message generated for change (Comment added) made by cgroves
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Category: None
Group: targeted for 2.2rc1
>Status: Closed
>Resolution: Fixed
Priority: 5
Private: No
Submitted By: Leonardo Soto (leosoto)
Assigned to: Nobody/Anonymous (nobody)
Summary: _weakref.ref(o) only works if the argument is hasheable

Initial Comment:
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.

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>Comment By: Charles Groves (cgroves)
Date: 2007-06-19 02:13

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This is fixed in r3259.

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Comment By: Lars Heuer (lheuer)
Date: 2007-05-11 05:06

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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




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Comment By: Charles Groves (cgroves)
Date: 2007-04-25 03:08

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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

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Comment By: Mr. Codepage (codepage)
Date: 2007-02-05 22:10

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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.

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Comment By: leouser (leouserz)
Date: 2007-01-15 19:20

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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.

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