[Python-3000] callable()

[Andrew Koenig]

> With your CD example, you need an external resource (the CD itself) in
> order to calculate the hash - in that case, you can't safely defer
> the hash calculation until the first time you know you need it,
> since you don't know whether or not you'll have access to the
> physical CD at that point.

OK.  Then instead of a CD, use a (read-only) disk file.  The example is
still valid in that case, and avoids the extraneous matter of operator
intervention.

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.