`cmath.nan` docs: Equivalent to `float('nan')`

[KardoPaska]

Documentation

Hello Friends!

Perhaps cmath.nan description can be more like math.nan?

Perhaps some insight to the intended nuances could benefit the description? Compare and contrast?

In the context of NaNs, the meaning of the word "equivalent" is quite nuanced.

Best!

https://docs.python.org/3/library/cmath.html#cmath.nan

cmath.nan
A floating-point “not a number” (NaN) value. Equivalent to float('nan').

Added in version 3.6.

https://docs.python.org/3/library/math.html#math.nan

math.nan
A floating-point “not a number” (NaN) value. Equivalent to the output of float('nan'). 
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 check whether a number is a NaN, use the 
[isnan()](https://docs.python.org/3/library/math.html#math.isnan) function to test for NaNs instead of is or ==. Example:

import math
math.nan == math.nan
False
float('nan') == float('nan')
False
math.isnan(math.nan)
True
math.isnan(float('nan'))
True
Added in version 3.5.

Changed in version 3.11: It is now always available.

Linked PRs

• gh-137871: Clarify cmath.nan documentation by linking to math module #137876
• [3.14] gh-137871: Clarify cmath.nan documentation by linking to math module (GH-137876) #140095
• [3.13] gh-137871: Clarify cmath.nan documentation by linking to math module (GH-137876) #140096

[AA-Turner]

cc @skirpichev

[skirpichev]

I think we could rather just add a reference to math.nan here:

A floating-point “not a number” (NaN) value. Equivalent to :data:math.nan.

Ditto for inf.