[Zope-dev] Please help - Persistent dictionary keys

[Martijn Pieters]

On Tue, Sep 19, 2000 at 09:41:57AM -0500, John D. Heintz wrote:
> Can someone please explain the best way for me to map from one instance
> to another?
> 
> In the following code either:
> (1) I am using dictionaries wrong, or
> (2) I'm missing something with ExtensionClass in general.  
> 
> I have tried searching Zope and Python mailing lists and resources for
> an answer to this question but have become confused by different
> conflicting messages.  On the one hand I have heard that dictionary keys
> are supposed to be immutable.  On the other, I've heard that
> dictionaries are used to simulate (key is object, value is ignored) some
> aspects of sets.  Which is correct?  
> 
> What I really want is to be able to map a Persistent object to a string
> with fast lookup.
> 
> Thanks for any help.
> John Heintz
> 
> 
> import ZODB
> from Persistence import Persistent
> 
> class Test1:
>     pass
> 
> class Test2(Persistent):
>     pass
> 
> dict = {}
> t1 = Test1()
> t2 = Test2()
> 
> try:
>     dict[t1] = 1
> except:
>     print "Using instance of Test1 as key failed!"
> 
> try:
>     dict[t2] = 2
> except:
>     print "Using instance of Test2(Persistent) as key failed!"

From the Python Library Reference:

  """A dictionary's keys are almost arbitrary values. The only types of
  values not acceptable as keys are values containing lists or
  dictionaries or other mutable types that are compared by value rather
  than by object identity."""

And from the Python Reference Manual:

  """__hash__ (self) 
  Called for the key object for dictionary operations, and by the built-in
  function hash(). Should return a 32-bit integer usable as a hash value
  for dictionary operations. The only required property is that objects
  which compare equal have the same hash value; it is advised to somehow
  mix together (e.g., using exclusive or) the hash values for the
  components of the object that also play a part in comparison of objects.
  If a class does not define a __cmp__() method it should not define a
  __hash__() operation either; if it defines __cmp__() but not __hash__()
  its instances will not be usable as dictionary keys. If a class defines
  mutable objects and implements a __cmp__() method it should not
  implement __hash__(), since the dictionary implementation requires that
  a key's hash value is immutable (if the object's hash value changes, it
  will be in the wrong hash bucket)."""

So, if you want to be able to use a Persistent based object as keys to a
dictionary, implement __cmp__ and __hash__ methods on that class:

>>> import ZODB
>>> from Persistence import Persistent
>>> class Test1:
...     pass   
... 
>>> class Test2(Persistent):
...   def __cmp__(self): return 1
...   def __hash__(self): return 1
... 
>>> dict = {}
>>> t1 = Test1()
>>> t2 = Test2()
>>> dict[t1] = 1
>>> dict[t2] = 2
>>> dict
{<Test2 instance at 80b3aa0>: 2, <__main__.Test1 instance at 80a3e78>: 1}
>>> 

-- 
Martijn Pieters
| Software Engineer  mailto:mj@digicool.com
| Digital Creations  http://www.digicool.com/
| Creators of Zope   http://www.zope.org/
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