[Python-Dev] Not-a-Number (was PyObject_RichCompareBool identity shortcut) (original) (raw)

Stephen J. Turnbull stephen at xemacs.org
Fri Apr 29 09:10:15 CEST 2011

Terry Reedy writes:

Python treats it as if it were a number:

As I said, so did the committee, and that was its mistake that we are more or less stuck with.

The committee didn't really have a choice. You could ask that they call NaNs something else, but some bit pattern is going to appear in the result register after each computation, and further operations may (try to) use that bit pattern. Seems reasonable to me to apply duck- typing and call those patterns "numbers" for the purpose of IEEE 754, and to define them in such a way that operating on them produces a non-NaN only when all numbers (including infinity) produce the same non-NaN.

The alternative is to raise an exception whenever a NaN would be generated (but something is still going to appear in the register; I don't know any number that should be put there, do you?) That is excessively punishing to Python users and programmers, though, since Python handles exceptions by terminating the computation. (Kahan points out that signaling NaNs are essentially never used for this reason.)

Other aspects of NaN behavior may be a mistake. But it's not clear to me, even after all the discussion in this thread.

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