Why are tuples immutable?

Antoon Pardon apardon at forel.vub.ac.be
Wed Dec 15 13:24:37 CET 2004


Op 2004-12-15, Fredrik Lundh schreef <fredrik at pythonware.com>:
> Antoon Pardon wrote:
>
>>> how would you implement a dictionary where the keys could change, without
>>> any performance penalty compared to the current implementation?
>>
>> The performace gained by using tuples as keys in dictionaries is
>> entirely illusional.
>>
>> Sure the fact that you use a tuple which is immutable, makes that
>> you can put the key directly in the dictionary instead of a copy
>> and that will gain you some performance.
>>
>> But this performance gain can be more than offset by other code
>> in the program.
>>
>> Suppose you need a list/tuple as a key and most opperation you
>> will do on those keys will be appends and pops. You now have the
>> itwo choices
>>
>>   1) Always convert your lists to tuples on key entries
>>   and keys accesses, which will mean more copying than when a
>>   copy of a key would have been made on key entry.
>>
>>   2) Simulate appends and pops by tuple operations which can also
>>   require more copying than was gained by using tuples as key
>
> sorry, but I don't understand your reply at all.  are you saying that dictionaries
> could support mutable keys (e.g lists) by making a copy of the key?  how would
> such a dictionary pick up changes to the original key object?  (I'm talking about
> the key stored in the dictionary, not the key you're using to look things up).

You want to mutate a key that is within a dictionary?

That sounds as bad as mutating an element in a sorted list.

-- 
Antoon Pardon



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