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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 get_monotone_constraints(self):
 """Get the monotone constraints of the Dataset. Returns ------- monotone_constraints : numpy array or None Monotone constraints: -1, 0 or 1, for each feature in the Dataset. """

        if self.monotone_constraints is None:
            self.monotone_constraints = self.get_field('monotone_constraints')
        return self.monotone_constraints