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Apr 2, 2022
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bpo-47031: Improve documentation for math.nan #32170

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a8006a7
Improve documentation for `math.nan`
CharlieZhao95 Mar 28, 2022
98275bd
Update Doc/library/math.rst
CharlieZhao95 Mar 29, 2022
cd3af79
Update Doc/library/math.rst
CharlieZhao95 Mar 29, 2022
cb51d82
remove examples related to `is`
CharlieZhao95 Mar 29, 2022
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23 changes: 21 additions & 2 deletions Doc/library/math.rst
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Expand Up @@ -646,8 +646,27 @@ Constants

.. data:: nan

A floating-point "not a number" (NaN) value. Equivalent to the output of
``float('nan')``.
A floating-point "not a number" (NaN) value. Equivalent to the output of
``float('nan')``. Specifically, due to the requirements of the `IEEE-754 standard
<https://en.wikipedia.org/wiki/IEEE_754>`_, ``math.nan`` and ``float('nan')`` are
not considered to equal to any other numeric value, including themselves. To avoid errors
when checking whether a number is a NaN, use the :func:`isnan` function to test
for NaNs instead of ``is`` or ``==``.
Example::

>>> import math
>>> math.nan == math.nan
False
>>> float('nan') == float('nan')
False
>>> math.nan is math.nan
True
>>> float('nan') is float('nan')
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Not too happy about this example because I could see us change this behavior in the future to return the same NaN object.

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@CharlieZhao95 CharlieZhao95 Mar 29, 2022 •
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Not too happy about this example because I could see us change this behavior in the future to return the same NaN object.

I feel the same way, this example is confusing. This phenomenon is simply due to the differences in function implementations, not related to any standard.

Maybe we should omit the example about is and just keep docs to remind users not use is to check if a number is a NaN.

False
>>> math.isnan(math.nan)
True
>>> math.isnan(float('nan'))
True

.. versionchanged:: 3.11
It is now always available.
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