How to distinguish different types of NaN float in Python

Question

I'm writing Python 2.6 code that interfaces with NI TestStand 4.2 via COM in Windows. I want to make a "NAN" value for a variable, but if I pass it float('nan'), TestStand displays it as IND.

Apparently TestStand distinguishes between floating point "IND" and "NAN" values. According to TestStand help:

• IND corresponds to Signaling NaN in Visual C++, while
• NAN corresponds to QuietNaN

That implies that Python's float('nan') is effectively a Signaling NaN when passed through COM. However, from what I've read about Signaling NaN, it seems that Signaling NaN is a bit "exotic" and Quiet NaN is your "regular" NaN. So I have my doubts that Python would be passing a Signaling NaN through COM. How could I find out if a Python float('nan') is passed through COM as a Signaling NaN or Quiet NaN, or maybe Indeterminate?

Is there any way to make a Signaling NaN versus QuietNaN or Indeterminate in Python, when interfacing with other languages? (Using ctypes perhaps?) I assume this would be a platform-specific solution, and I'd accept that in this case.

Update: In the TestStand sequence editor, I tried making two variables, one set to NAN and the other set to IND. Then I saved it to a file. Then I opened the file and read each variable using Python. In both cases, Python reads them as a nan float.

Answer 1

I dug a bit for you, and I think you might be able to use the struct module in combination with the information on at Kevin's Summary Charts. They explain the exact bit patterns used for the various kinds of IEEE 754 floating point numbers.

The only thing you probably will have to be careful for, if I read the topics on this IND-eterminate value, is that that value tends to trigger some kind of floating point interrupt when assigned directly in C code, causing it to be turned into a plain NaN. Which in turn meant those people were advised to do this kind of thing in ASM rather than C since C abstracted that stuff away.. Since it is not my field, and that I am not sure to what extent this kind of value would mess with Python, I figured I'd mention it so you can at least keep an eye for any such weird behaviour. (See the accepted answer for this question).

>>> import struct

>>> struct.pack(">d", float('nan')).encode("hex_codec")
'fff8000000000000'

>>> import scipy
>>> struct.pack(">d", scipy.nan).encode("hex_codec")
'7ff8000000000000'

Referring to Kevin's Summary Charts, that shows that float('nan') is actually technically the Indeterminate value, while scipy.nan is a Quiet NaN.

Let's try making a Signaling NaN, and then verify it.

>>> try_signaling_nan = struct.unpack(">d", "\x7f\xf0\x00\x00\x00\x00\x00\x01")[0]
>>> struct.pack(">d", try_signaling_nan).encode("hex_codec")
'7ff8000000000001'

No, the Signaling NaN gets converted to a Quiet NaN.

Now let's try making a Quiet NaN directly, and then verify it.

>>> try_quiet_nan = struct.unpack(">d", "\x7f\xf8\x00\x00\x00\x00\x00\x00")[0]
>>> struct.pack(">d", try_quiet_nan).encode("hex_codec")
'7ff8000000000000'

So that's how to make a proper Quiet NaN using struct.unpack()--at least, on a Windows platform.