Why do “Not a Number” values equal True when cast as boolean in Python/Numpy?

This is in no way NumPy-specific, but is consistent with how Python treats NaNs:

In [1]: bool(float('nan'))
Out[1]: True

The rules are spelled out in the documentation.

I think it could be reasonably argued that the truth value of NaN should be False. However, this is not how the language works right now.