On Thu, 19 Apr 2007 17:40:27 -0400, John Bauman wrote:
> Adam Atlas wrote:
>> On Apr 19, 5:24 pm, Bill Jackson <[EMAIL PROTECTED]> wrote:
>>> I have a dictionary of dictionaries where the keys are typically very
>>> long tuples and repeated in each inner dictionary. The dictionary
>>> representation is nice because it handles sparseness well...and it is
>>> nice to be able to look up values based on a string rather than a
>>> number. However, since my keys are quite long, I worry that I am
>>> wasting a lot of memory.
>>
>> I wouldn't worry about it. Try doing hash('string_2') in the
>> interpreter -- the output thereof is what's really being used as the
>> key. It doesn't use up any more memory than the integer 2.
>>
> Are you sure about that? Most dictionaries need to store the actual key,
> in case of a collision, so when you lookup a key they can tell which
> you're really looking for.
The key is already stored, so long as the string 'string_2' exists. And it
will continue to exist so long as the dictionary includes it as a key. An
extra copy isn't made (unless you make an extra copy yourself).
In other words, the dictionary stores a reference to the string, not a
copy of it:
>>> s = "this is a long string"
>>> d = {s: 1}
>>> id(s)
-1209073840
>>> id(d.keys()[0])
-1209073840
--
Steven.
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