multi-Singleton-like using __new__

[Matt Nordhoff]

Steven D'Aprano wrote:
> Except that using has_key() means making an attribute lookup, which takes 
> time.

I was going to say that, but doesn't 'in' require an attribute lookup of
some sort too, of __contains__ or whatever? has_key is probably now just
a wrapper around that, so it would be one more attribute lookup. What's
the difference in performance between attribute lookups from Python code
and from the internal C code?

> Now, if you want to argue that the difference between 0.3 microseconds 
> and 0.2 microseconds is insignificant, I'd agree with you -- for a single 
> lookup. But if you have a loop where you're doing large numbers of 
> lookups, using in will be a significant optimization.

I meant that it's (usually) insignificant, and barely larger than the
usual inaccuracy due to my system's load (I'm tired; what's the word for
that?).
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