nans and object identity

Description

nans (like float("nan"), np.nan) follow the IEEE 754 standard for floating-point arithmetic, which specifies that NaN is not equal to anything, including itself, so:

np.nan == np.nan  # False
float("nan") == float("nan") # False

But

np.nan is np.nan  # True

So these are objects that are not equal but identical to themselves. This leads to unexpected results in cases where object identity is taken as a short cut before equality comparisons, e.g. in tuples.

Snippet preview

>>> import numpy as np
>>> np.nan == np.nan
False
>>> (np.nan, np.nan) == (np.nan, np.nan)
True
>>> nan1 = float('nan'); nan2 = float('nan')
>>> nan1 == nan2
False
>>> (nan1, nan2) == (nan1, nan2)
True
>>> (nan1, nan2) == (nan2, nan1)
False

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