@cache_readonly
    def _isnan(self):
        if self._can_hold_na:
            return isnull(self)
        else:
            # shouldn't reach to this condition by checking hasnans beforehand
            values = np.empty(len(self), dtype=np.bool_)
            values.fill(False)
            return values
return if each value is nan
dataset/ETHPy150Open pydata/pandas/pandas/indexes/base.py/Index._isnan