Why are tuples immutable?

Antoon Pardon apardon at forel.vub.ac.be
Thu Dec 16 12:25:31 CET 2004


Op 2004-12-16, Fredrik Lundh schreef <fredrik at pythonware.com>:
> Antoon Pardon wrote:
>
>> That depends on whether the programmes wants value equality
>> or identity equality.
>>
>> In the first case the programmer shouldn't mutate a after
>> it was introduced as key in the dictionary; but should
>> either introduce a copy or work on a copy later. As
>> such your snippet of code would become.
>>
>>  a = [1,2,3]
>>  d[a[:]] = 9
>>  a.append(4)
>>  print d[a]
>>
>> And this would raise a KeyError, unless the list [1,2,3,4]
>> would be in the dictionary.
>>
>> In the second case your code would produce 9.
>
> how does the dictionary know if you want key value equality or key
> identity equality?

By the __hash__ and __eq__ methods you provide on your object.

-- 
Antoon Pardon



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