[Baypiggies] identity cache / flywheel design pattern

Shannon -jj Behrens jjinux at gmail.com
Sun Aug 3 03:57:23 CEST 2008

I've blogged about it here:

> i wonder if you have a newbie nugget here
> regardless of subsequent improvements: would you please explain what's
> going on in the two setdefault() statements that makes this work?
> what's being returned to s1 and then to s2?

The code is basically saying:

if identity_cache.has_key(s2):
    s2 = identity_cache[s2]  # Get the cached instance of the same value.
    identity_cache[s2] = s2  # Cache the instance.

s2 always has the same "value", but the above code makes sure there's
only *one copy* of that value.

> It sounds like a job for the "intern" built-in function

Bingo.  Right you are!  It makes the identity_cache completely

> Curious that your signature referred to Lisp, which has used the
> intern concept for decades! ;-)

Proof positive that there's a lot about Lisp I don't know ;)

> BTW the pattern name is 'flyweight'

You know, when I read "flyweight" in the gang of four, I envisioned a
flywheel, and I could never figure out why they picked that name ;)

> But maybe that's still flyweight, just a really simple form of it.

I remember the example in the book was using a flyweight for character
objects.  Hence, every 'a' would be the same instance.

Thanks guys!

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