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2010-10-11T06:50:09.953
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2010-10-13T00:58:45.870
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2010-10-13T00:58:45.870
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60075
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3886988
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CPython definition of nan When Python reports a <code>nan</code>, where does that come from? <ul> <li>Result of a calculation (platform specific values?)</li> <li><code>Py_NAN</code> in the CPython C source code <ul> <li>defined as <code>(Py_HUGE_VAL * 0.)</code> <ul> <li>Value is platform-specific</li> <li><code>Py_HUGE_VAL</code> is probably defined as <code>HUGE_VAL</code>--it has a note to say it should be <code>HUGE_VAL</code> except on platforms where that is broken.</li> </ul></li> </ul></li> <li><code>float('nan')</code> which is defined from <code>Py_NAN</code> in CPython's C source code.</li> </ul> Reading Python and pywin32 Source Code I've had a look at the C source code for <code>pywin32</code>, in particular <code>win32com</code>, which forms the Python↔COM translation layer. That code: <ul> <li>takes the input object</li> <li>calls <code>PyNumber_Float()</code> to convert it to a Python <code>float</code> (if it isn't already)</li> <li>calls <code>PyFloat_AsDouble()</code> to convert it to a plain C <code>double</code> value. <ul> <li>This simply returns the C <code>double</code> directly contained in the <code>PyFloatObject</code> member <code>ob_fval</code>.</li> </ul></li> </ul> So it looks as though I've traced a <code>NaN</code> from the COM interface back to a plain C <code>double</code> type containing <code>Py_NAN</code>, whatever that turns out to be on the Windows platform. TestStand NAN Value Now I've tried this with NI TestStand. First I tried: <pre><code>quiet_nan = struct.unpack(">d", "\x7f\xf8\x00\x00\x00\x00\x00\x01")[0] # Set the variable's value in TestStand locals_prop_object.SetValNumber(var_name, 0, quiet_nan) </code></pre> But that still appeared in TestStand as <code>IND</code>. So then I created a TestStand file with variables set to <code>IND</code> and <code>NAN</code>, and read the values from Python. It turns out that TestStand's <code>NAN</code> has a value of <code>FFFF000000000001</code>. According to <a href="http://babbage.cs.qc.edu/courses/cs341/IEEE-754references.html#kevin_chart" rel="nofollow">Kevin's Summary Charts</a> that is a negative quiet NAN. TestStand's <code>IND</code> does have the expected value for Indeterminate, <code>FFF8000000000000</code>. Success So, after all that, I have succeeded in setting a NAN in TestStand, from Python: <pre><code># Make a NAN suitable for TestStand teststand_nan = struct.unpack(">d", "\xff\xff\x00\x00\x00\x00\x00\x01")[0] # Set the variable's value in TestStand locals_prop_object.SetValNumber(var_name, 0, teststand_nan) </code></pre>
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1. USCraig McQueen
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