[Python-Dev] redefining is
Ilya Sandler
ilya at bluefir.net
Thu Mar 18 12:53:25 EST 2004
> > The observation is that if an object is immutable, there's
> > no legitimate reason to know whether it's distinct from
> > another object with the same type and value.
>
Here is a real life example where "is" on immutable objects is very
helpful for perfomance reasons
I have DatabaseRecord objects which have a field "schema":
(which comes from Database objects)
schema is just a string with textual description of database schema and
this string might be quite long (like 500-700 bytes)
given 2 databaserecord objects I want to be able to quickly say whether
they have the same schema..(with time cost being independent on number of
field, record size, etc)
It's easy to achieve with is, I just intern schemas:
database.schema=intern(schema)
dbRec1.schema=database.schema
dbRec2.schema=database.schame
(Essentially, intern() is done once per database open() operation)
then comparing 2 schemas is much quicker done with "is"
(dbRec1.schema is dbRec3.schema)
than with either id() or "=="
Or am I missing something here?
Ilya
PS and just for reference some timings:
src>./python Lib/timeit.py -n 10000 -s 'l=500; x="1" * l; y="1"* l' 'x is y'
10000 loops, best of 3: 0.143 usec per loop
src>./python Lib/timeit.py -n 10000 -s 'l=500; x="1" * l; y="1"* l' 'x == y'
10000 loops, best of 3: 0.932 usec per loop
src>./python Lib/timeit.py -n 10000 -s 'l=500; x="1" * l; y="1"* l' 'id(x) == id(y)'
10000 loops, best of 3: 0.482 usec per loop
s
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