[Python-Dev] gc ideas -- sparse memory (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Dec 4 20:37:14 CET 2010

Why is useful to expose an identity hash? AFAICS it is only useful in building an identity hash table. If so, why not just provide id() or the is operator or both and be done with it?

That's precisely James' point: Java provides the identity hash instead of the id() function (i.e. it does not have an equivalent of id()). Doing so gives greater liberties in implementing Java. For example, an implementation that would only use the type and not the instance for identity hash would still be conforming (as would one that always returns 0).

With identity, I don't see much point in leaving that up to the programmer. In implementations with stable object addresses, the address is a fine implementation of identityhash() -- perfect, in fact.

Of course. James' complaint is that Python-the-language mandates support for an id() function, though - a requirement that even implementations that don't have stable object addresses now must support. If Python mandated only identity hash properties of the id() function, alternative implementations could be simplified.

ISTM that's no problem, you have to do an extra comparison for identity when the codes are equal either way. Of course the identity hash code is a more precise optimization, but that doesn't make hash() unusable for this purpose (cf Sun's built-in implementation of IdentityHashCode that always returns 2).

That's only the case if the hash() result is guaranteed not to change. In some applications, it may be desirable to have that as an absolute guarantee (rather than just being a convention).

No, you can't substitute identity hash with hash: the value hash of an object may change over time, whereas the identity hash must not.

Regards, Martin

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