Kurz mal hier getestet, da ich kein numpy installiert habe:
Python 2.7.6 (default, Mar 22 2014, 22:59:56)
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In [1]: import numpy as np
In [2]: np.float64(56.9)
Out[2]: 56.899999999999999
In [3]: np.float64(56.9) == float('nan')
Out[3]: False
In [4]: np.float64(-57.9) == float('nan')
Out[4]: False
In [5]: float(-57.9) == float('nan')
Out[5]: False
In [6]: float(56.9) == float('nan')
Out[6]: False