object identity and hashing

Steve Holden steve at holdenweb.com
Mon Feb 25 15:21:30 CET 2008


castironpi at gmail.com wrote:
> On Feb 24, 9:28 pm, George Sakkis <george.sak... at gmail.com> wrote:
>> On Feb 24, 9:11 pm, castiro... at gmail.com wrote:
>>
>>
>>
>>
>>
>>> On Feb 24, 7:58 pm, Jeff Schwab <j... at schwabcenter.com> wrote:
>>>> castiro... at gmail.com wrote:
>>>>> Can someone explain this?
>>>>>>>> a= {}
>>>> Create an empty dict and bind it to the name a.
>>>>>>>> a[(3,)]= 0
>>>> Set the key/value pair (3,):0 to the dict.
>>>>>>>> (3,) in a
>>>> Is (3,) one of the keys in the dict?
>>>>> True
>>>> Yes, it is.
>>>>>>>> (3,) is (3,)
>>>> Create two separate tuples (that happen to be equivalent).  Are they the
>>>> same object?
>>>>> False
>>>> No, they are not.
>>>> Every time you write (3,), you are potentially creating a new object.
>>>> These objects have equal values (and hash codes), so they are
>>>> interchangeable for purposes of keying a dict.
>>> I see.  You stated,
>>>> Is (3,) one of the keys in the dict?
>>>>> True
>>>> Yes, it is.
>>> It isn't, but it does equal a key that's already in the dict.
>> Right, dict lookup depends on object hashing (__hash__) and equality
>> (__eq__), not identity. There are legitimate cases where lookup should
>> be based on identity [1], but they are far less common than those
>> based on equality.
>>
>> George
>>
>> [1]http://www.martinfowler.com/eaaCatalog/identityMap.html- Hide quoted text -
>>
>> - Show quoted text -
> 
> [1] illustrates a case in which 'a is a' returns False, and in the
> other corner of the DeMorgan table, there is 'a is b' returns True for
> 'a == b' False.
But this doesn't tell you anything about Python except that it's 
flexible enough to construct counter-intuitive classes.

Everything you have been told is true for the normal cases you will come 
across in everyday usage. If you want to play in the obscure corners of 
the language that's fine, but don't try and treat them as mainstream.

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC              http://www.holdenweb.com/




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