[Python-Dev] PyObject_RichCompareBool identity shortcut (original) (raw)

Steven D'Aprano steve at pearwood.info
Fri Apr 29 03:37:16 CEST 2011

Nick Coghlan wrote:

1. IEEE754 is a value-based system, with a finite number of distinct NaN payloads 2. Python is an object-based system. In addition to their payload, NaN objects are further distinguished by their identity (infinite in theory, in practice limited by available memory).

I argue that's an implementation detail that makes no difference. NANs should compare unequal, including to itself. That's the clear intention of IEEE-754. There's no exception made for "unless y is another name for x". If there was, NANs would be reflexive, and we wouldn't be having this discussion, but the non-reflexivity of NANs is intended behaviour.

The clear equivalent to object identity in value-languages is memory location. If you compare variable x to the same x, IEEE754 says you should get False.

Consider:

Two distinct NANs are calculated somewhere...

x = float('nan') y = float('nan')

They find themselves in some data in some arbitrary place

seq = [1, 2, x, y] random.shuffle(seq)

And later x is compared to some arbitrary element in the data

if math.isnan(x): if x == seq[0]: print("Definitely not a NAN")

nan != x is an important invariant, breaking it just makes NANs more complicated and less useful. Tests will need to be written "if x == y and not math.isnan(x)" to avoid getting the wrong result for NANs.

I don't see what the problem is that we're trying to fix. If containers wish to define container equality as taking identity into account, good for the container. Document it and move on, but please don't touch floats.

-- Steven

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