[Python-3000] callable()

[Giovanni Bajo]

Andrew Koenig wrote:

> It's still the case that evaluating a hash might be expensive the
> first time you do it, so you may want to be able to test whether it
> can be done and actually do it only later.

I think you're doing a lot of guess-work here. In real world situations,
computing the hash is very very fast by design (at worse O(n) as you noticed
before for the string case, but a very fast algorithm with a very low
constant multiplier), it is done only once, and checking if the whole object
is (recursively) hashable is *very* near to the actual cost of computing the
hash. Simple immutable objects have simple, fast hash functions; large
mutable objects use id(). An object with a very long string can be an
exception, but I don't think this situation is common enough to warrant a
special-case and a complication of the language.

I still need to hear one good reason to have a (performance critical!) check
of the *hashability* of an object, in the first place. I never ever had to
do it, let alone in a critical path of my code.
-- 
Giovanni Bajo