[Python-Dev] redefining is (original) (raw)

Tim Peters tim.one at comcast.net
Sat Mar 20 01:39:42 EST 2004

[Guido]

Sorry, if you're usign any immutable value there and expecting it to be a unique object, you're cruisin' for a bruisin', so to speak. The language spec explicitly allows but does not require the implementation to cache and reuse immutable values.

[Andrew]

Ay, there's the rub.

Aren't you saying that using "is" to compare immutables is always broken, unless you know that the immutable values are singletons?

No, because it's still the purpose of "is" to expose the implementation. Examples of system-level uses were given earlier; for another, if I'm doing a crawl over gc.get_referrers(x) trying to account for all the references to the object currently bound to x (this is something I do often when trying to track suspected memory leaks), object identity is the only notion of equivalence that's relevant, and it doesn't matter whether that object is a singleton, or mutable, neither any other property of x's type or behavior -- the only thing about x that matters in this context is its indenty, and locating which other objects refer to exactly it. If I know that references to the string "<oid at 00000001>" are growing over time, I'll in fact add an "is" test against that string to the guts of Zope's test.py leak-detection scaffolding. It's necessary to be able to spell this in Python -- although it's not necessary (IMO) to be able to spell it with two letters.

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