[Python-Dev] Not-a-Number (original) (raw)

Robert Kern robert.kern at gmail.com
Fri Apr 29 17:31:00 CEST 2011

On 4/29/11 1:35 AM, Nick Coghlan wrote:

On Fri, Apr 29, 2011 at 3:28 PM, Steven D'Aprano<steve at pearwood.info> wrote:

Robert Kern wrote:

Actually, Python treats all NaNs as quiet NaNs and never signalling NaNs.

Sorry, did I get that backwards? I thought it was signalling NANs that cause a signal (in Python terms, an exception)? If I do x = 0.0/0 I get an exception instead of a NAN. Hence a signalling NAN. Aside from the divide-by-zero case, we treat NaNs as quiet NaNs.

And in fact, 0.0/0.0 is covered by the more general rule that x/0.0 raises ZeroDivisionError, not a rule that converts IEEE-754 INVALID exceptions into Python exceptions. Other operations that produce a NaN and issue an IEEE-754 INVALID signal do not raise a Python exception.

But that's not the difference between signalling NaNs and quiet NaNs. A signalling NaN is one that when it is used as an input to an operation, it issues an INVALID signal, not whether a signal is issued when it is the output of an operation.

-- Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco

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