The IEEE-754 requirement that NANs are never equal to anything, even to themselves, is a common stumbling block for those new to the consequences of IEEE-754. See for example #47020.

The documentation for math.nan would be a good place to add a note like

"Due to the requirements of the `IEEE-754 standard <https://en.wikipedia.org/wiki/IEEE_754>`_, math.nan and float('nan') are never equal to any other value, including themselves. Use math.isnan to test for NANs."

https://docs.python.org/3.8/library/math.html#math.nan