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Mon Feb 16 15:15:16 EST 2004 · https://mail.python.org/pipermail/python-dev/2004-February/042583.html

[Martin von Löwis]
> A mutable type is one whose instances have state that can change over
> the life of the object. [...]

Interesting presentation.  The following detail is worth stressing:

>   equal objects should hash equal.

This should be stressed as a formal requirement for a correctly behaved
hashing function.  The other formal requirement being that a hash
function should always compute the same value for the lifespan of an
object, and that value should be a non-negative integer (not a long
one?).  A _desirable_ property, but not a requirement, is that two
unequal objects yield different hashes.

So, instead of explaining how we should cleverly use `__hash__' to
describe immutability, we might describe what `__hash__' ought to
be, and then merely notice that hashable objects could be useful to
represent immutable objects (whether user objects or builtin objects).

-- 
François Pinard   http://www.iro.umontreal.ca/~pinard

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[Python-Dev] bool does not want to be subclassed?