dictionary keys, __hash__, __cmp__

Michael Hudson mwh at python.net
Wed Nov 5 14:16:58 CET 2003


Jan-Erik Meyer-Lütgens  <python at meyer-luetgens.de> writes:

> In the Python Language Reference, I found the following
> statements about using objects as dictionary keys:
> 
>     1. "__hash__() should return a 32-bit integer."
> 
>     2. "The only required property is that objects which
>         compare equal have the same hash value."
> 
>     3. "If a class does not define a __cmp__() method it
>         should not define a __hash__() operation either."
> 
> 
> Can I asume that:
> 
>   -- it is guaranteed that id(obj) returns a unique 32-bit integer

No.  For instance it doesn't on a 64-bit platform (and on a ridiculous
64 bit platform *cough*Win64*cough* it might even return a long).

>   -- keys are interchangeable (equivalent),
>      if the following is valid:
> 
>          hash(key1) == hash(key2) and key1 == key2

Not quite sure what you mean by equivalent, but if that is true
a key inserted under key1 can be retrieved with key2.

>   -- I can ignore the 2nd statement, if I am aware of
>      the fact that: if objects are equal it dosn't mean that
>      they are the same key.

Um, I don't understand you, but I think the answer is "no".

If you ever have a situation where hash(a) != hash(b) but a == b then
you are very much breaking the rules.

>   -- I can savely ignore the 3rd statement, because python
>      falls back to cmp(id(obj1), id(obj2)), if __cmp__()
>      is not defined.

Don't understand.

Cheers,
mwh

-- 
  I wouldn't trust the Anglo-Saxons for much anything else.  Given
  they way English is spelled, who could trust them on _anything_ that
  had to do with writing things down, anyway?
                                        -- Erik Naggum, comp.lang.lisp




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