How can I create customized classes that have similar properties as 'str'?

Steven D'Aprano steve at
Sun Nov 25 02:12:32 CET 2007

On Sun, 25 Nov 2007 01:38:51 +0100, Hrvoje Niksic wrote:

> samwyse <samwyse at> writes:
>> create a hash that maps your keys to themselves, then use the values of
>> that hash as your keys.
> The "atom" function you describe already exists under the name "intern".

Not really. intern() works very differently, because it can tie itself to 
the Python internals. Samwyse's atom() function doesn't, and so has no 

In any case, I'm not sure that intern() actually will solve the OP's 
problem, even assuming it is a real and not imaginary problem. According 
to the docs, intern()'s purpose is to speed up dictionary lookups, not to 
save memory. I suspect that if it does save memory, it will be by 

>From the docs:

intern(  string)
Enter string in the table of ``interned'' strings and return the interned 
string - which is string itself or a copy. Interning strings is useful to 
gain a little performance on dictionary lookup - if the keys in a 
dictionary are interned, and the lookup key is interned, the key 
comparisons (after hashing) can be done by a pointer compare instead of a 
string compare. Normally, the names used in Python programs are 
automatically interned, and the dictionaries used to hold module, class 
or instance attributes have interned keys. Changed in version 2.3: 
Interned strings are not immortal (like they used to be in Python 2.2 and 
before); you must keep a reference to the return value of intern() around 
to benefit from it.

Note the words "which is string itself or a copy". It would be ironic if 
the OP uses intern to avoid having copies of strings, and ends up with 
even more copies than if he didn't bother.

I guess he'll actually need to measure his memory consumption and see 
whether he actually has a memory problem or not, right?


More information about the Python-list mailing list