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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def std(self) -> Optional[float]: #, ddof=0):
 """Standard deviation of all values entered into histogram. This number is precise, because we keep the necessary data separate from bin contents. Returns ------- float """

        # TODO: Add DOF
        if self._stats:
            return np.sqrt(self.variance())
        else:
            return None